CENAPRED reported that each day during 31 October-6 November there were 89-192 steam-and-gas emissions from Popocatépetl, some of which contained minor amounts of ash. Periods of volcanic tremor were detected almost daily. Explosions at 1638 and 1727 on 3 November ejected material NE and generated plumes that rose 1.5 and 1.6 km above the crater rim, respectively. The Alert Level remained at Yellow, Phase Two (middle level on a three-color scale).

Ongoing steam, gas, and ash emissions along with intermittent explosions, August 2017-February 2018

Located 60 km SE of Mexico City, frequent historical eruptions have been reported from Popocatépetl going back to the 14th century. Activity increased in the mid-1990s after about 50 years of quiescence, and the current eruption, which has been ongoing since January 2005, has included frequent ash plumes and numerous episodes of lava-dome growth and destruction within the 500-m-wide summit caldera. Multiple emissions of steam and gas occur daily, rising generally 1-4 km above the 5.4-km-elevation summit; many contain small amounts of ash. Larger, more explosive events that generate ashfall in neighboring communities often occur every week.

Activity through July 2017 was typical of the ongoing eruption with near-constant emissions of water vapor, gas, and minor ash, as well as multiple explosions every week with ash-plumes and incandescent blocks sent down the flanks (BGVN 42:09). This report covers similar activity through February 2018. Information about Popocatépetl comes from daily reports provided by México's Centro Nacional de Prevención de Desastres (CENAPRED); ash emissions are also reported by the Washington Volcanic Ash Advisory Center (VAAC). Satellite visible and thermal imagery and SO2 data also provide important observations.

Near-constant emissions of steam and gas, often with minor ash content, were typical activity for throughout August 2017-February 2018. Intermittent larger explosions with plumes of moderate ash content that generated ashfall in nearby communities were reported in most months, including several times during October and November 2017, reaching communities as far as 70 km away. Incandescence at the summit was often observed on clear nights, and Strombolian activity that sent incandescent blocks several hundred meters down the flanks occurred at least once each month during September 2017-January 2018. The tallest ash plumes during the period reached 9.1 km altitude in mid-October and 10.3 km altitude at the end of January 2018. Thermal anomalies were persistently detected in satellite data throughout the period, and SO2 plumes were recorded every month with satellite instruments.

Activity during August-September 2017. The Washington VAAC reported satellite observations of an ash plume extending 55 km W of the summit at 6.4 km altitude on 31 July 2017; the plume was mostly gas and steam with a small amount of ash. CENAPRED reported ashfall in Ozumba (18 km W) on 1 August from a plume that rose 2 km above the summit. They also noted numerous low-intensity explosions with steam, gas, and ash during 5-7 August. A small explosion early on 14 August produced a 500-m-high plume with minor ash content that drifted SW. Two explosions later in the day generated ash plumes that rose 0.8 and 1.5 km from the summit and drifted W (figure 94). Another explosion on 15 August produced a plume over 1 km in height with moderate ash content. On 21 August CENAPRED reported an ash plume that rose 4 km and drifted NW (figure 95). The Washington VAAC reported this plume extending 33 km W from the summit at 7.6 km altitude. Later in the day the ash cloud was observed about 230 km W of the summit, and a new cloud at a slightly lower altitude had drifted 45 km NW.

Figure 94. An ash plume drifted W from Popocatépetl on 14 August 2017 as seen from the Tlamacas webcam located about 5 km N of the volcano. Courtesy of CENAPRED.

Figure 95. An ash plume at Popocatépetl rose 4 km above the summit on 21 August 2017 and drifted over 200 km W before dissipating. View is from the Altzomoni webcam, located about 10 km N of the summit. Courtesy of CENAPRED.

CENAPRED noted 22 explosions with ash during 25-26 August that drifted N and NW. They were observed in satellite imagery by the Washington VAAC at 7.6 km altitude. Eleven explosions with small amounts of ash were reported by CENAPRED on 27 August. There were daily explosions during 28-31 August, but weather clouds obscured views of the summit. Incandescence at the summit crater was observed on many clear nights during August.

During 1-11 September 2017 cloudy conditions generally prohibited observations of the summit, but low-intensity emissions of steam and gas were briefly observed, many containing minor ash. Five explosions with minor ash emissions were reported by CENAPRED on 12 September; the Washington VAAC noted the ash plume in satellite imagery at 6.7 km altitude drifting slowly N. CENAPRED reported 22 explosions with ash and incandescent rocks on the NE flank during 12-13 September.

The Washington VAAC reported ash plumes on 13 September at 8.2 km altitude, on 18 September at 6.4 km altitude drifting W, and on 23 September near 7 km altitude moving to the NNE. Numerous explosions were reported by CENAPRED during 27 and 28 September (figure 96). The Washington VAAC reported the dense ash plume from these explosions at 6.7 km altitude drifting WSW. It extended 130 km W of the volcano by early afternoon on 27 September. CENAPRED reported that an explosion late on 30 September sent incandescent fragments 0.8 km from the crater and produced a dense ash column that rose more than 2 km above the summit.

Figure 96. A dense ash emission from Popocatépetl on 27 September 2017 extended 130 km W before dissipating as viewed from the Altzomoni webcam, located about 10 km N of the summit. Courtesy of CENAPRED.

Activity during October-November 2017. The ash plume from the explosion late on 30 September 2017 was visible in satellite imagery the following morning located 15 km SW from the summit at 7.9 km altitude according to the Washington VAAC. CENAPRED reported three explosions on 2 October and five explosions the next day, causing ashfall in Atlautla (17 km W), Tepetlixpa (21 km W), and Ozumba. Three explosions on 5 October resulted in ashfall in Totolapan (32 km W), Tlalnepantla (40 km W), and Cuernavaca (64 km W), and closer to the volcano in Ecatzingo (15 km SW), Atlautla, and Tepetlixpa. Lahars were also observed on the W flank, but there were no reports of damage. Two more explosions on 6 October led to ashfall reported from Zacualpan de Amilpas (30 km SW) and Tetela del volcán (18 km SW) (figure 97). The Washington VAAC reported the 6 October emissions at 6.4 km altitude.

Figure 97. Webcam image showing one of the two explosions on 6 October 2017 at Popocatépetl that caused ashfall in Zacualpan de Amilpas (30 km SW) and Tetela del volcán (18 km SW). The Tlamacas webcam is located about 5 km N of the volcano. Courtesy of CENAPRED.

The first of two explosions on 7 October 2017, shortly after midnight, produced a plume that rose over 2 km and drifted SW with ashfall reported in Tetela del volcán; incandescent blocks were also sent down the flanks (figure 98). The second explosion produced an ash plume that rose 3 km and drifted NNE. The Washington VAAC reported continuing ash emissions during 7-11 October. Numerous plumes rose to 5.8-9.1 km altitude and drifted in several different directions; the plume extended 130 km SW from the summit on 10 October. CENAPRED reported three explosions on 8 October (figure 99) and two on 9 October. Numerous low-intensity exhalative events during 10-12 October produced ash plumes less than 1 km above the crater that drifted SW. Ashfall was reported in several communities during this time including Ozumba, México City (60 km NW), Milpa Alta (45 km NW), Xochimilco (56 km NW), Tlalpan (68 km NW), Coyoacán (66 km NW), Iztapalapa (57 km NW), Magdalena Contreras (72 km NW), and Iztacalco (64 km NW).

Figure 98. Incandescent blocks visible in this image traveled down the flanks of Popocatépetl during the early morning of 7 October 2017. The Tlamacas webcam is located about 5 km N of the volcano. Courtesy of CENAPRED.

Figure 99. Multiple explosions from Popocatépetl on 8 October 2017, including the one seen here, caused ashfall in several communities NW of the volcano. The Tlamacas webcam is located about 5 km N of the volcano. Courtesy of CENAPRED.

CENAPRED noted incandescence at the crater during most nights from 14 to 31 October, as well as steam, gas, and minor ash from hundreds of low-intensity emission events each day. The Washington VAAC reported ash emissions visible in satellite imagery on 16, 20-22, and 26 October drifting in several different directions at altitudes of 5.8-7.6 km. The plume observed on 22 October reached 60 km from the summit before dissipating. CENAPRED reported two explosions with ash plumes each day during 25-27 October. The Washington VAAC reported an ash plume on 29 October at 6.1 km altitude drifting E about 35 km from the summit, and another at 6.7 km the following day along with an infrared hotspot visible at the summit.

The Washington VAAC issued multiple daily ash advisories throughout November 2017. CENAPRED reported hundreds of daily low intensity emissions of gas and steam that often contained minor ash; the plumes generally rose about 1 km above the summit and most often drifted SW. They also observed incandescence at the crater on all clear nights. They reported Strombolian activity on 3 November in the early morning that lasted for several hours. Explosions early on 4 November resulted in minor ashfall in Yecapixtla (29 km SW) and Zacualpan de Amilpas and other areas to the SW. A Strombolian episode later that day lasted for about an hour and resulted in minor ashfall in Tetela del Volcán. Another explosion that night sent incandescent fragments 200 m down the flanks.

An explosion on 6 November sent an ash plume 2.5 km above the summit crater that drifted SW and sent incandescent fragments 500 m down the flank. Another explosion during the early morning of 7 November produced a 2-km-high ash plume. Moderate amounts of ash rose 1 km above the summit on 8 November. There were three explosions on 10 November; the largest produced a 3-km-high ash plume that drifted SW. Continuous low-level emission of gas and ash on 14 November resulted in ashfall reported in Totolapan, Yecapixtla, Ocuituco (23 km SW), Tetela del Volcán, and Ecatzingo. An explosion on 17 November sent an ash plume 2.5 km above the summit that drifted SW. During 18-19 November five explosions caused ash plumes to rise 2 km above the summit and incandescent blocks to fall down the E flank.

Around 1030 on 20 November, seismic activity increased and was accompanied by a constant plume of steam, gas, and moderate ash that rose about 1.5 km and drifted E. During 20-21 November eight explosions were reported, with five more the following day. During the afternoon of 23 November a continuous ash emission that lasted 90 minutes drifted SSE at 2 km above the summit, and spread ash over communities to the SSE including Huaquechula (30 km SSE), Tepeojuma (38 km SE), Atlixco (23 km SE), and Izúcar de Matamoros (50 km SE) (figure 100). Another significant ash emission during the afternoon of 24 November sent a column of ash to 4 km above the summit, drifting SSE; it lasted for almost two hours (figure 101). The Washington VAAC reported the plume at 8.5 km altitude. Ashfall was reported in San Pedro Benito Juárez (12 km SE) and Atlixco. Late that evening, an explosion sent incandescent fragments 1 km down the flanks and generated an ash plume that rose to 2.5 km above the summit and also drifted SSE.

Figure 100. A continuous ash emission at Popocatépetl that lasted for 90 minutes drifted SSE at 2 km above the summit, and spread ash over several communities to the SSE on 23 November 2017. The Tlamacas webcam is located about 5 km N of the volcano. Courtesy of CENAPRED.

Figure 101. A substantial ash emission at Popocatépetl during the afternoon of 24 November 2017 sent a column of ash to 4 km above the summit that drifted SSE; it lasted for almost two hours. The Washington VAAC reported the plume at 8.5 km altitude. The Altzomoni webcam is located about 10 km N of the summit. Courtesy of CENAPRED.

A flyover by CENAPRED and the Federal Police on 25 November 2017 allowed evaluation of the changes in the summit crater from the recent explosions. They noted that the internal crater within the summit crater had increased its dimensions, reaching a diameter of 370 m and a depth of 110 m (figure 102). A 3-km-tall ash plume resulted from continuous emissions that began in the afternoon of 27 November and lasted for two hours. The Washington VAAC reported the plume at 7.9 km altitude. The plume drifted SSE, and dispersed ash over communities in that region including Tochimilco (16 km), Izucar de Matamoros, Atlixco, and Huaquechula.

Figure 102. During a flyover on 25 November 2017, CENAPRED observed that the increased size of the internal summit crater at Popocatépetl was 370 m in diameter and 110 m deep. Courtesy of CENAPRED.

Activity during December 2017-February 2018. The Washington VAAC issued multiple daily reports of ash emissions during 1-12 and 24-31 December 2017. CENAPRED noted hundreds of daily low-intensity emissions of gas and steam, most with small quantities of ash, throughout December, as well as multiple ash emissions on many days that rose generally 1-2.5 km above the summit. In the early morning of 2 December an explosion caused an ash plume to rise 2.5 km above the summit. A second plume rose 1 km later that day; they both drifted SSE. An explosion in the afternoon of 9 December sent an ash plume over 2.5 km above the summit that drifted NE. The Washington VAAC reported the plume at 7.6 km altitude. Later that evening Strombolian activity sent incandescent blocks down the flanks and generated an ash plume that drifted E. Incandescence was observed at the summit crater during the nights of 17-21 and 24-29 December. Continuous emissions of steam, gas, and moderate-density ash were reported drifting NW for about 90 minutes on 29 December. An explosion on 31 December at 1032 generated a 2-km-high ash plume that also drifted NW.

There were multiple daily reports of ash emissions issued by the Washington VAAC during most days of January 2018. CENAPRED noted hundreds of daily low-intensity emissions of gas and steam, many with small quantities of ash, throughout the month, as well as explosions with ash emissions on many days that generally rose 1-2.5 km above the summit. They also observed incandescence at the summit crater multiple days each week. Ongoing low-level emissions of steam, gas, and minor ash were reported during 4-5 January. During the evening of 5 January activity increased, and the ash plume rose to 800 m and drifted SE. In addition, incandescent blocks were ejected 200-300 m down the flanks for about two hours.

An explosion on 18 January 2018 generated an ash plume that rose 1.5 km above the summit and drifted E while incandescent blocks were ejected up to 700 m down the flanks. An episode of Strombolian activity in the early morning of 25 January produced an ash plume that rose 2 km above the summit and drifted N and NE, resulting in reports of ashfall in San Pedro Nexapa (14 km NE) and Amecameca (19 km NE). It lasted for about 2 hours. Four explosions were reported during the afternoon of 29 January and an explosion the following afternoon produced an ash plume that rose more than 3 km above the summit, and was dispersed to the NW. An explosion on 31 January also produced a substantial ash plume that the Washington VAAC reported at 10.3 km altitude moving NNE (figure 103).

Figure 103. An ash plume rose to 10.3 km altitude from Popocatépetl on 31 January 2018 and drifted NNE. The Altzomoni webcam is located about 10 km N of the summit. Courtesy of CENAPRED.

Activity was somewhat quieter at Popocatépetl during February 2018. The Washington VAAC reported ash emissions on 14 days during the month. CENAPRED reported tens, not hundreds, of daily low-intensity emissions of gas and steam that often contained minor amounts of ash. They also noted one or more explosions with ash emissions on many days that rose generally 1-1.5 km above the summit and drifted in various directions. During many clear days they observed nearly constant emissions of steam, gas, and minor ash that reached 500-800 m above the summit. An explosion on 20 February produced an ash plume that rose 1.5 km above the summit. Continuous steam and gas emissions during 22-23 February were accompanied by minor incandescence intermittently observed at the summit.

Satellite data. Sulfur dioxide emissions were large enough to be recorded by satellite instruments several times every month during August 2017-February 2018 (figure 104). Variable wind directions and persistent emissions produced relatively long-lived plumes that dispersed over large areas of Mexico.

Thermal anomaly data provided by the MIROVA project are consistent with the visual record of persistent incandescent and explosive activity at the summit (figure 105). Multiple MODVOLC thermal alerts were also recorded every month from October 2017-February 2018.

Figure 105. Thermal anomalies detected by satellite-based MODIS instruments and recorded through the MIROVA project show the pattern of continued moderate-level activity at Popocatépetl during the year ending 12 July 2018. Courtesy of MIROVA.

CENAPRED reported that each day during 31 October-6 November there were 89-192 steam-and-gas emissions from Popocatépetl, some of which contained minor amounts of ash. Periods of volcanic tremor were detected almost daily. Explosions at 1638 and 1727 on 3 November ejected material NE and generated plumes that rose 1.5 and 1.6 km above the crater rim, respectively. The Alert Level remained at Yellow, Phase Two (middle level on a three-color scale).

CENAPRED reported that each day during 12-17 September there were 64-189 steam-and-gas emissions from Popocatépetl, some of which contained minor amounts of ash. Nighttime crater incandescence was sometimes visible. Explosions were detected almost every day: eight on 12 September; one on 14 September; five on 15 September; three on 16 September. A series of emissions and explosions accompanied by tremor began at 0425 on 17 September and lasted for 365 minutes; incandescent tephra was ejected. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that each day during 25-31 July there were 24-42 steam-and-gas emissions from Popocatépetl, and nightly crater incandescence. Explosions were detected almost every day: five on 26 July; nine on 27 July; one on 28 July; three on 30 July. A series of gas-and-ash emissions began at 0307 on 31 July and lasted for 215 minutes. Ejected incandescent tephra landed on the flanks. The gas-and-ash plumes rose 2 km above the crater rim and drifted WSW, causing ashfall in Tetela del Volcán, Yecapixtla, Tlalnepantla, and Totolapan (Morelos state), and in Amecameca, Acuautla, Ecatzingo, Ozumba, and Tepetlixpa (Mexico state). The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that each day during 6-12 June there were 19-34 steam-and-gas emissions from Popocatépetl, and nightly crater incandescence. Explosions were detected almost every day: at 2026 on 7 June; 0130 on 8 June; 1756, 1931, and 2358 on 9 June; 1724 on 10 June. An explosion at 0220 on 11 June ejected incandescent fragments. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that each day during 9-15 May there were 51-137 steam and gas emissions from Popocatépetl as well as ongoing incandescence from the summit. Additionally, three explosions were recorded: at 1834 on 11 May, at 0912 on 11 May, and at 1452 on 14 May. These explosions dispersed ash to the S and SW. Volcano-tectonic earthquakes with magnitudes up to M 2.8 also occurred throughout the time period.

CENAPRED reported that each day during 25 April-1 May there were 63-114 steam and gas emissions from Popocatépetl, often containing ash. Incandescence from the crater was visible at night. As many as five explosions per day were recorded during 25-29 April, with resulting eruption plumes rising around 1 km above the crater rim. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during an overflight of Popocatépetl on 16 March scientists observed a small lava dome, number 78, at the bottom of the inner crater. The dome was 50 m in diameter and 30 m thick, and produced gas plumes visible above the main crater rim. The inner crater was 320 m in diameter and about 100 m deep; remnants of the previous dome had been deposited on the walls of the inner crater. Each day during 21-27 March there were 20-233 emissions, often containing slight amounts of ash. Incandescence from the crater was visible at night. Plumes of gas and water vapor drifted WSW, SSW, SSE, and SE. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that each day during 7-13 March there were 105-361 steam and gas emissions from Popocatépetl. Incandescence from the crater was visible at night. Explosions were recorded on 7 March. An explosion at 1042 on 12 March generated an ash plume that rose 1.5 km above the crater rim and drifted SE. The Alert Level remained at Yellow, Phase Two.

Each day during 7-13 February CENAPRED reported 25-101 emissions from Popocatépetl, with emissions during 11-13 February containing slight amounts of ash. Explosions were detected at 0130 and 2213 on 7 February, at 0457 on 8 February, at 1729 on 12 February, and at 0631 on 13 February. Minor crater incandescence was visible on the morning of 9 February, and at night during 11-12 February. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that at 1348 on 1 January an explosion at Popocatépetl generated a plume with low ash content that rose 0.5 km above the crater rim and drifted SE. Each day during 3-9 January there were 110-588 emissions, some of which contained minor amounts of ash. Beginning at 2032 on 6 January Strombolian activity generated an ash, gas, and water vapor emission that rose 800 m and drifted SE, and ejected incandescent material 200-300 m onto the flanks. Explosions were recorded at 0131 and 0222 on 8 January. The Alert Level remained at Yellow, Phase Two.

Each day during 22-28 November CENAPRED reported 188-725 emissions from Popocatépetl, and as many as five explosions. Two explosions on 23 November produced minor ashfall in the municipalities of Huaquechula (30 km SSW), Tepeojuma (39 km SE), Atlixco (23 km SE), and Izúcar de Matamoros (50 km SSE), in the state of Puebla. After one explosion (at 1413) there was a 90-minute period of emissions. After an explosion at 0512 on 24 November (the second of five recorded that day) a 108-minute-period of emissions was recorded. Minor amounts of ash fell in Atlixco. Almost two hours of continuous emissions of gas, steam, and ash began at 1711, producing a plume that rose as high as 4 km above the crater rim and drifted SSE. Ashfall was reported in San Pedro Benito Juárez (10-12 km SE) and Atlixco, in the state of Puebla. An explosion at 2252 ejected incandescent fragments as far as 1 km from the crater. An ash plume rose 2.5 km and drifted SSE. Two periods of emissions were recorded on 25 November, at 1110 (lasting 132 minutes) and 1929 (lasting 35 minutes). During an overflight that day observers noted that recent explosive activity had increased the dimensions of the internal crater (the crater on the main crater floor) to 370 m in diameter and 110 m deep. A 121-minute-long period of emissions began at 1529 on 27 November, with plumes rising at least 3 km and drifting SSE. The Alert Level remained at Yellow, Phase Two.

Each day during 1-7 November CENAPRED reported 200-361 emissions from Popocatépetl, some of which contained ash. Crater incandescence was noted almost nightly. Increased seismicity at 0146 on 3 November coincided with a period of Strombolian activity that ended at 0535; gas, water vapor, and ash emissions rose from the carter and incandescent material was ejected 500 m onto the flanks. An explosion was detected at 1027. On 4 November explosions were detected at 0145 and 0608. Following the second explosion a continuous gas plume with minor ash drifted WSW. Beginning at 0735 ashfall was reported in municipalities of Yecapixtla (31 km SW) and Zacualpan de Amilpas (30 km SW), in the state of Morelos. A Strombolian period began at 1029 and lasted for 80 minutes, causing ashfall in Tetela del Volcán (20 km SW). Explosions at 1440 and 2231 ejected incandescent material 200 m onto the flanks. Another explosion was recorded at 0411 on 5 November. An explosion at 1653 on 6 November generated a gas-and-ash plume that rose 2.5 km above the crater rim and drifted SW. Fragments were ejected 500 m onto the flanks. An explosion at 0100 on 7 November generated a gas-and-ash plume that rose almost 2 km above the crater rim and drifted SW. The Alert Level remained at Yellow, Phase Two.

Each day during 26 September-3 October CENAPRED reported 21-61 emissions from Popocatépetl and 2-6 volcano-tectonic earthquakes. Cloud cover often prevented visual observations. Periods of harmonic tremor and as many as 10 explosions per day were detected during 26-30 September. Beginning at 0315 on 27 September an episode of Strombolian activity that lasted for six hours and sixteen minutes ejected incandescent tephra as far as 1 km onto the flanks. Ash plumes rose 1.5 km above the crater rim and drifted W. Ash fell in the towns of Ecatzingo and Atlautla in Estado de México, and in Atlatlahuacán, Ocuituco, Oaxtepec, Jiutepec, and Yautepec in Estado de Morelos. An explosion at 2257 on 30 September ejected incandescent tephra no more than 800 m onto the flanks, and produced an ash plume that rose 2 km. An explosion was detected at 1417 on 3 October. The Alert Level remained at Yellow, Phase Two. CENAPRED stated that there was no significant increase in activity at Popocatépetl related to the M 7.1 earthquake, centered beneath Puebla (45 km E), that occurred at 1314 on 19 September.

Each day during 12-19 September CENAPRED reported 141-299 steam and gas emissions from Popocatépetl. Cloud cover often prevented observations, though gas-and-steam plumes were visible daily. During 12-13 September there were 22 explosions detected, four of which generated emissions with minor amounts of ash and ejected incandescent tephra. An explosion was detected at 1820 on 14 September. On 19 September a plume with low ash content rose 1 km. CENAPRED stated that there was no significant increase in activity at Popocatépetl related to the M 7.1 earthquake, centered beneath Puebla (45 km E), that occurred at 1314. The Alert Level remained at Yellow, Phase Two.

Each day during 9-13 and 15 August CENAPRED reported 97-355 steam and gas emissions from Popocatépetl; the daily count increased to 702 on 14 June. Crater incandescence was visible on some nights. A small explosion at 0815 on 14 August produced a plume with low ash content that rose 500 m above the crater rim and drifted SW. Explosions at 1759 and 1805 generated ash plumes that rose 0.8 and 1.5 km, respectively, and drifted W. On 15 August an explosion produced an ash plume that rose 1 km and drifted WNW. The Alert Level remained at Yellow, Phase Two.

Each day during 28 June-4 July CENAPRED reported 67-240 steam and gas emissions from Popocatépetl, some of which contained minor amounts of ash. Explosions were detected on 28 June (4), on 30 June (1), on 2 July (5), and on 3 July (1), though cloudy conditions prevented visual confirmation of possible ash, gas, and steam plumes. Minor ashfall on 2 July was noted in Ozumba, Amecameca, Tlalmanalco, Chalco, Ayapango, Tenango del Aire, and San Pedro Nexapa. An explosion at 1145 on 4 July generated an ash plume that rose 2.5 km above the crater rim and drifted W. The Alert Level remained at Yellow, Phase Two.

Each day during 31 May-2 June CENAPRED reported 51-78 steam and gas emissions from Popocatépetl; the daily count increased to 144-276 during 2-5 June. Explosions were detected during 31 May-1 June (1-2 per day) and during 2-5 June (13-22 per day) though cloudy conditions prevented visual confirmation of ash, gas, and steam plumes. Observers noted material being ejected 200 m from the crater on 3 June. Crater incandescence was visible on some nights. The Alert Level remained at Yellow, Phase Two.

Each day during 25 April-2 May CENAPRED reported 26-109 steam and gas emissions from Popocatépetl, sometimes containing ash, and crater incandescence at night or in the early morning. Around seven explosions were detected almost each day, with as many as 15 counted during 26-27 April (from mid-morning to mid-morning). Explosions generated plumes that consisted of water vapor and gas, with low ash content, and during 26-27 April they ejected tephra as far as 100 m NE of the crater. The Alert Level remained at Yellow, Phase Two.

Each day during 8-14 March CENAPRED reported 87-200 steam and gas emissions from Popocatépetl, and crater incandescence on most nights. Weather clouds often prevented visual observations. Explosions were detected during 8-11 March: at 0809 on 8 March, at 1847 on 9 March, at 0539 on 10 March, and at 0435 on 11 March. Two additional explosions on 11 March, at 1347 and 1842, generated ash plumes that rose less than 2 km above the crater rim and drifted ENE. The Alert Level remained at Yellow, Phase Two.

Each day during 22-28 November CENAPRED reported 129-324 steam and gas emissions from Popocatépetl that sometimes contained ash. Volcano-tectonic events were detected during 22-24 November, and explosions occurred on 22 and 24 November (4 and 7 events, respectively). At 0945 on 25 November an explosion generated a plume that rose 5 km above the crater rim and drifted SE and NE. Seismicity decreased after the event. Ashfall was reported in areas downwind including in the municipalities of Atlixco, Tochimilco, and San Pedro Benito Juárez. During 28-29 November there were 48 detected emissions. Beginning at 0559 emissions of water vapor, gas, and ash became constant, rising as high as 1.5 km above the crater rim and drifting NE. Incandescent fragments were ejected 300-800 m form the crater, mainly onto the NE flank. Ash fell in Atlixco, Chiautzingo, Domingo Arenas, Huejotzingo, Juan C. Bonilla, San Andrés Calpan, and San Martín Texmelucan (Puebla state), and in San Miguel (Tlaxcala state). The phase of continuous emissions and ejected material ended at 1630 on 30 November. The Alert Level remained at Yellow, Phase Two.

Each day during 2-8 November CENAPRED reported 40-185 steam, gas, and ash emissions from Popocatépetl. Crater incandescence was visible nightly, and at dawn on 2 November. The Alert Level remained at Yellow, Phase Two.

Each day during 28 September-4 October CENAPRED reported 125-389 steam, gas, and ash emissions from Popocatépetl. Cloud cover often prevented observations, though gas-and-steam plumes were visible daily. Crater incandescence was visible on some nights. An explosion at 0929 on 29 September produced a plume that drifted NW. Explosions were also detected at 1813 on 30 September, 1300 on 3 October, and 0231 and 0647 on 4 October. The Alert Level remained at Yellow, Phase Two.

During an overflight of Popocatépetl on 30 August CENAPRED scientists confirmed that explosions during 27-28 August had destroyed lava dome 69 (first identified on 1 August). The crater which had hosted the dome was 300 m in diameter and 30 m deep.

Each day during 7-13 September there were 35-133 emissions, some of which contained minor amounts of ash on 8 September. Cloud cover sometimes prevented observations, though gas-and-steam plumes were visible daily. Crater incandescence was visible at night and sometimes was more intense in conjunction with emissions. An explosion at 1450 on 8 September produced an ash plume that rose 1.5 km above the crater. On 11 September an explosion at 0925 generated a plume that rose 1 km, and an explosion at 2323 ejected incandescent material onto the flanks. The Alert Level remained at Yellow, Phase Two.

Each day during 10-16 August CENAPRED reported 35-133 emissions from Popocatépetl, some of which contained minor amounts of ash, and as many as four explosions. Cloud cover sometimes prevented observations, though gas-and-steam plumes were visible almost daily. Crater incandescence was visible at night. On 11 August there were six landslides detected by the seismic network; the largest one occurred on the NW flank at 0853 and had a volume of 440 m3, and the second largest one, on the N flank, occurred at 1756 and had a volume of 220 m3. An explosion on 12 August generated an ash plume that rose 2.5 km above the crater and drifted WNW, causing ashfall in Ozumba (18 km W) and Atlautla (16 km W). An explosion at 0034 on 13 August ejected incandescent material onto the flanks. Two more explosions that day and one on 14 August produced plumes with low ash content that rose as high as 1 km. The Alert Level remained at Yellow, Phase Two.

During 20-26 July CENAPRED reported 26-182 daily emissions from Popocatépetl, some of which contained minor amounts of ash. Cloud cover often prevented observations, though gas-and-steam plumes were visible almost daily. Crater incandescence was visible on some nights. Explosions were detected during 24-26 July: 4 on 24 July, 4 on 25 July, and 1 on 26 July. The Alert Level remained at Yellow, Phase Two.

During 29 June-5 July the seismic network at Popocatépetl detected 128-193 daily emissions and as many as five daily explosions. Cloud cover often prevented observations, although crater incandescence was visible every night. Explosions at 1348 and 1405 on 4 July produced ash plumes that rose 1.5 and 1.2 km above the crater, respectively. Ashfall was reported in Atlatlahucan (30 km WSW) and Tepetlixpa (20 km W). The Alert Level remained at Yellow, Phase Two.

During 8-14 June there were 47-104 daily emissions from Popocatépetl and as many as six explosions detected daily; some emissions contained minor amounts of ash. Cloud cover often prevented observations, though crater incandescence was visible every night. During 0638-1130 on 9 June continuous ash emissions rose as high as 1 km above the crater rim and drifted NE. An explosion at 1117 on 12 June produced an ash plume that rose 2.5 km and drifted W. Minor amounts of ash fell in Ozumba (18 km W). The Alert Level remained at Yellow, Phase Two.

During 1-7 June there were 94-251 daily emissions from Popocatépetl and as many as four explosions detected daily; some emissions corresponded with increased crater incandescence at night. A series of explosions detected during 1135-2230 on 4 June generated gas-and-steam emissions with minor amounts of ash that rose 1.5 km above the crater and drifted ENE. Some of the explosions ejected incandescent tephra 300 m onto the NE flanks. An explosion on 5 June generated an ash plume that rose 1.5 km and drifted E. Incandescent tephra was ejected 1 km onto the N flank. Between 0354 and 0824 on 6 June continuous explosions formed plumes that rose 1 km and drifted NE. The Alert Level remained at Yellow, Phase Two.

During 18-24 May there were 43-492 daily emissions from Popocatépetl and as many as eight explosions detected daily; some emissions corresponded with increased crater incandescence at night. Periods of low-amplitude harmonic tremor were also detected almost daily. Daily cloud cover prevented visual observations. The Alert Level remained at Yellow, Phase Two.

During 13-17 April CENAPRED reported 10-105 emissions from Popocatépetl and as many as two explosions detected daily; some emissions corresponded with increased crater incandescence. Activity increased at 0232 on 18 April. Strombolian activity ejected incandescent fragments 1.6 km onto the NE flank, and ash plumes rose 3 km above the crater and drifted ENE. Ashfall was reported in San Pedro Benito Juárez, San Nicolás de los Ranchos, Tianguismanalco, San Martín Texmelucan, and Huejotzingo. Activity decreased over the next few hours; ash plumes rose 1 km and drifted ENE. According to a news article, the airport in Puebla closed due to the ash plumes. During 18-19 April CENAPRED noted 103 emissions an intense crater incandescence. The Alert Level remained at Yellow, Phase Two.

During 6-12 April there were 38-136 emissions from Popocatépetl and as many as six explosions detected daily; some emissions corresponded with increased crater incandescence. Periods of low-amplitude harmonic tremor were detected during 6-8 April. An explosion at 0111 on 9 April produced a gas-and-ash plume that rose 1.5 km above the crater and drifted NE, and ejected incandescent fragments 300 m away onto the E flank. The Alert Level remained at Yellow, Phase Two.

At 1200 on 30 March CENAPRED reported that during the previous 24 hours the seismic network at Popocatépetl recorded 179 emissions and four explosions. Ash plumes from the explosions rose as high as 3.5 km above the crater; ash from the explosions caused Puebla's airport to close from 2000 on 29 March to 0600 on 30 March.

During 30 March-5 April there were 38-136 emissions and as many as six explosions detected daily; some emissions corresponded with increased crater incandescence. An explosion at 0103 on 31 March produced an ash plume that rose 1.8 km and drifted ENE, and ejected incandescent fragments 1 km away onto the ESE flank. An explosion at 1521 generated an ash plume that rose 2 km and drifted ENE. With the assistance of the Federal Police, on 2 April CENAPRED scientists conducted an overflight of the crater and observed an inner crater that was 325 m in diameter and 50 m deep; the crater had previously been filled with a lava dome, destroyed in January, which had grown to an estimated volume of 2,000,000 cubic meters. Small landslides had occurred on the E wall of the inner crater. An explosion at 2031 on 3 April generated an ash plume that rose 2 km and drifted NE. Incandescent fragments were ejected as far as 3.5 km onto the E and SE flanks, generating fires in that part of the forest. Ash fell in the towns of Juan C. Bonilla (32 km ENE) and Coronango (35 km ENE), both in the state of Puebla. SINAPRED noted that the explosion was the strongest recorded in the previous three years. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 22-29 March the seismic network at Popocatépetl recorded 6-60 daily emissions that sometimes contained ash; 127 were detected on 28 March. Crater incandescence was observed on most nights. At 1052 on 24 March an ash plume rose 1.6 km above the crater and drifted NE. At 0026 the next morning a low-intensity explosion generated an ash plume that rose 500 m and drifted NE. Incandescent tephra was ejected as far as 400 m onto the N flank. Explosions during 27-29 March generated plumes that rose as high as 1.5 km. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 10-16 February the seismic network at Popocatépetl recorded 21-88 daily emissions consisting of water vapor, gas, and ash. Crater incandescence was noted on some nights and increased in intensity with some emissions. Explosions occurred almost daily; an explosion at 2320 on 11 February ejected incandescent tephra onto the NE flank. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 2-9 February the seismic network at Popocatépetl recorded 17-83 daily emissions consisting of water vapor, gas, and ash, and as many as nine explosions per day. Ash plumes during 6-7 February rose 700-800 m above the crater and drifted E. Crater incandescence was noted most nights and increased in intensity with some emissions. Explosions at 1630 and 1709 on 8 February generated ash plumes that rose 1 km and drifted ENE. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 20-23 and 25 January the seismic network at Popocatépetl recorded 16-68 daily emissions consisting of water vapor, gas, and ash, and 2-5 explosions per day. At 1000 on 23 January an increase in activity was characterized by continuous gas-and-ash emissions, likely related to the destruction of a recently-formed lava dome. Later that night cameras recorded incandescent fragments ejected during periods of emissions. Activity decreased at 0200 on 24 January and then increased again at 0844, with explosions. A series of explosions on 25 January began at 1000 and ended at 1840. Constant steam-and-ash emissions drifted ENE. Another series of explosions occurred during 0218-0600 on 26 January. One of the explosions ejected incandescent fragments 900 m onto the NE flank. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 29 December-5 January the seismic network at Popocatépetl recorded 4-92 daily emissions; 191 were detected on 3 January. As many as 11 explosions were detected daily and variable nighttime crater incandescence was observed. Explosions on 2 January ejected tephra onto the N flank, and a small steam, gas, and ash plume drifted E on 4 January. A seven-hour period of explosions started at 2246 on 4 January and ended at 0545 on 5 January. Six more explosions occurred in the evening. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 11-17 November the seismic network at Popocatépetl recorded 30-62 daily emissions; 102 and 88 were detected on 14 and 16 November, respectively. Variable nighttime crater incandescence was observed on some days. The seismic and acoustic network registered explosions almost daily. Ash plumes rose from the crater on 12 November; ash plumes rose 2 km on 17 November and incandescent material was deposited on the flanks within 1 km of the crater. Daily gas plumes drifted SW and NW. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 21-22 and 24-27 October the seismic network at Popocatépetl recorded 12-69 daily emissions; 128 were detected on 23 October. Cloud cover often prevented visual observations. Variable nighttime crater incandescence was observed on some days. Two, four, and six explosions were detected on 21, 22, and 23 October, respectively. Four explosions were detected during 26-27 October. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 30 September-6 October the seismic network at Popocatépetl recorded 23-142 daily emissions consisting of water vapor, gas, and sometimes ash; cloud cover often prevented visual observations. Variable nighttime or morning crater incandescence was observed most days, and 1-7 daily explosions were registered. On 3 October a gas, steam, and ash plume rose 2 km and drifted NW. During a series of explosions on 6 October, material was ejected onto the N flank, not far from the crater. Gas, steam, and ash plumes drifted NE. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 16-22 September the seismic network at Popocatépetl recorded 15-89 daily emissions consisting of water vapor, gas, and sometimes ash; cloud cover often prevented visual observations. Variable nighttime or morning crater incandescence was observed most days, and 1-13 daily explosions were registered. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 22-28 July the seismic network at Popocatépetl recorded 34-102 daily emissions consisting of water vapor, gas, and sometimes ash; cloud cover often prevented visual observations. Variable nighttime crater incandescence was observed, and explosions were detected almost daily. Gas-and-ash plumes rose 1.5 km on 26 July, and two explosions on 27 July produced ash plumes that rose less than 1 km. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 15-21 July the seismic network at Popocatépetl recorded 18-64 daily emissions consisting of water vapor, gas, and sometimes ash; cloud cover often prevented visual observations. Variable nighttime crater incandescence was observed, and explosions were detected during 17-19 July. Two explosions on 20 July produced ash plumes that rose less than 500 m. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 1-7 July the seismic network at Popocatépetl recorded 18-118 daily emissions consisting of water vapor, gas, with ash seen on 7 July only; cloud cover often prevented visual observations. Gas-and-steam plumes were observed daily, and variable nighttime crater incandescence was observed. An explosion was detected at 1024 on 1 July, and 14 were detected during 5-7 July. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 27 May-2 June the seismic network at Popocatépetl recorded 60-145 daily emissions consisting of water vapor, gas, and ash; cloud cover sometimes prevented visual observations. Explosions occurred daily, and nighttime crater incandescence was observed. Gas-and-steam plumes drifted in multiple directions. On 29 May at 1600 a series of explosions generated steam-and-ash plumes, and ashfall in multiple municipalities. Another series of explosions was detected from 1600-1918 on 30 May. Slight ashfall was recorded in Amozoc, Puebla (60 km E) on 31 May. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 20-26 May the seismic network at Popocatépetl recorded 40-307 daily emissions consisting of water vapor, gas, and sometimes ash; cloud cover often prevented visual observations. Nighttime crater incandescence was noted every night; sometimes the incandescence would become more intense with accompanying emissions. A small explosion at 0023 on 21 May generated a plume with low ash content that rose 800 m and drifted SW. During 22-24 May ash plumes rose 0.5-2.5 km and drifted SW. Ashfall was reported in Ocuituco (24 km SW) on 22 May. From 1702 to 1955 on 25 May a series of explosions accompanied by tremor ejected steam, gas, and ash plumes that drifted SSE. Explosions were detected on 26 May; ashfall was rpeorted in Tetela del Volcán (20 km SW).The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 6-12 May the seismic network at Popocatépetl recorded 17-56 daily emissions mostly consisting of water vapor and gas. Nighttime crater incandescence was noted almost every night; sometimes the incandescence would become more intense with accompanying emissions. Explosions were detected at 0949 and 1113 on 7 May. The next day steam-and-gas emissions with low ash content drifted SSW. An explosion was detected at 0411. Ash was visible in water vapor-and-gas emissions during 10-11 May. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 8-14 April the seismic network recorded 13-77 gas and steam emissions, with another 204 emissions recorded over 1-2 May. Ash accompanied the emissions during 1-3 May. Gas-and-steam plumes were visible, although cloud cover mostly prevented observations. Nighttime crater incandescence was often noted. A series of explosions during 2218-2301 on 30 April ejected incandescent tephra 200 m onto the NE flank. Sequences of explosions were also detected during 0758-1356 on 1 May and during 0411-0935 on 2 May. Ashfall was reported in San Pedro Benito Juárez (10-12 km SE) in the municipality of Atlixco Puebla on 2 May. Explosions were also detected during 3-5 May. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 22-28 April the seismic network at Popocatépetl recorded 25-91 daily emissions mostly consisting of water vapor and gas. Cloud cover sometimes prevented observations of the crater, although gas plumes and nighttime crater incandescence were noted daily. On 22 April an explosion at 0121 produced diffuse gas and water vapor emissions. Explosions at 1643 and 1758 generated ash plumes. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 15-21 April the seismic network at Popocatépetl recorded 19-157 daily emissions. Cloud cover sometimes prevented observations of the crater, although ash plumes and nighttime crater incandescence were often noted. Explosions at 0617 and 0857 on 15 April generated ash plumes that rose 1 km and drifted E. On 17 April an explosion was detected as well as a steam-and-gas emission with low ash content that rose 1-2 km. The next day, on 18 April, six explosions generated steam-and-gas plumes with small amounts of ash that rose as high as 1.5 km and drifted NE. A series of smaller, low-intensity explosions between 1636 and 2330 produced emissions of steam, gas, and small amounts of ash that rose 300 m and drifted NE. Some incandescent tephra fell 100-500 m away onto the N and NE flanks. On 19 April seven explosions generated steam-and-gas plumes with small amounts of ash that rose as high as 1.5 km and drifted NE. At 1052 on 20 April an explosion produced an ash plume that rose 3 km and drifted E. Incandescent tephra was ejected 500 m E. On 21 April three explosions generated plumes with some ash that rose 500 m.

CENAPRED reported that after a series of explosions ended at 1200 on 7 April the seismic network at Popocatépetl recorded 78 low-intensity emissions through 1100 on 8 April; gas-and-steam plumes containing small amounts of ash drifted SE. Also during this period 93 explosions occurred, and 12 minutes of harmonic tremor were detected on 8 April. During 8-14 April the seismic network recorded 20-112 gas, steam, and ash emissions, and nighttime crater incandescence was often noted. On 9 and 10 April the network detected 41 and 120 minutes of harmonic tremor, respectively. During an overflight on 10 April scientists confirmed that a lava dome was emplaced in the bottom of the crater between 24 March and 4 April. The lava dome was at least 250 m in diameter and 30 m thick. The surface of the dome had concentric fractures and the central part was collapsed from deflation. Explosions were detected during 13-14 April. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 1-7 April the seismic network at Popocatépetl recorded 101-191 daily emissions except during 3-4 and 6-7 April when only 37 and 53 were detected, respectively. Cloud cover often prevented observations of the crater, although ash plumes and nighttime crater incandescence were often noted. On 3 April multiple ash plumes rose 1-3 km above the crater. A period of harmonic tremor, that began at 1039 and ended at 1338, was accompanied by continuous emissions of steam with small amounts of ash. During 3-4 April explosions generated ash plumes that rose as high as 2 km; ashfall was reported in Tetela and Ocuituco. More explosions on 4 April generated ash plumes that rose 0.5-2 km and drifted SW. During 4-6 April explosions ejected incandescent tephra 100-700 m onto the flanks, and produced steam, gas, and ash plumes that rose 1.2-2 km and sometimes drifted W. Episodes of tremor were detected on 5 April. At 0754 on 7 April an episode of explosions and tremor began. Steam, gas, and ash plumes rose from the crater and material was ejected short distances from the crater. The Alert Level remained at Yellow, Phase Two.

At 2113 on 24 March activity at Popocatépetl increased and a four-hour series of explosions produced steam, gas, and ash emissions that rose 3 km. Incandescent tephra was ejected 800 m onto the NE and SE flanks. The last explosion in the series was detected at 0118 on 25 March. Additional explosion on 25 March ejected tephra and generated steam, gas, and ash plumes; the plumes rose 2 km and drifted NE and SE causing ashfall in Atlixco, Puebla. During 26-31 March the seismic network recorded between 21 and 86 emissions per day that sometimes contained ash. Cloud cover often prevented observations of the crater, although ash plumes and nighttime crater incandescence were noted. Continuous steam, gas, and ash emissions on 26 March rose 600 m and drifted ENE. On 29 March at 0944 an ash plume rose 2 km. On 31 March one of three explosions, which occurred at 0740, generated an ash plume that rose less than 1 km and drifted SW. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 18-24 March the seismic network at Popocatépetl recorded 90-222 daily emissions and 1-13 explosions. Cloud cover often prevented observations of the crater, although ash plumes and nighttime crater incandescence were noted. At 0344 on 22 March an explosion ejected incandescent tephra onto the flanks and produced an ash plume that rose 1 km above the crater rim and drifted NE. An ash plume rose 1 km and drifted SE on 23 March. An ash plume rose 1 km on 24 March. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 10-17 March the seismic network at Popocatépetl recorded between 6 and 100 gas-and-steam emissions that likely contained some ash; cloud cover mostly prevented observations of the volcano, so on most days ash in the plumes was not confirmed. Two explosions at 1047 on 10 March generated ash plumes that rose 1.5 km above the crater. An explosion on 11 March produced a plume that rose 1 km. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 3-10 March the seismic network at Popocatépetl recorded between 39 and 211 gas, steam, and ash emissions. Crater incandescence at night sometimes increased coincident with emissions. During 3-4 March there were 45 explosions detected; an explosion at 2004 on 3 March ejected tephra 700 m from the crater onto the NE flank, and other explosions ejected material 100 m onto the flanks. Gas-and-ash plumes drifted NE. On 5 March there were 29 explosions. Ash plumes mostly rose less than 1 km, although one gas, steam, and ash plume rose 3 km and drifted NE. The next day an explosion generated an ash plume that rose 1.2 km and drifted NE. A series of explosions on 7 March produced ash plumes that rose 1-2 km and drifted SW and N. Incandescent tephra was ejected 800 m onto the flanks and ashfall was reported in Ecatzingo (15 km SW). A series of explosions on 8 March produced ash plumes that rose 2 km and drifted NE. Ashfall was reported in Amecameca (20 km NW), Ecatzingo (15 km SW), and Tepextlipa. On 9 March a low-intensity explosion detected at 1014 generated a water vapor, gas, and ash plume. On 10 March two explosions were detected (at 0747 and 0859). The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that a series of explosions at Popocatépetl from 2250 on 24 February to 0345 on 25 February was accompanied by periods of tremor and Strombolian activity which ejected incandescent material as far as 700 m onto the NE and SE flanks. Additional explosions (19) were detected on 25 February. Ashfall was reported in San Martín Texmelucan, San Matías Tlalancaleca, San Salvador el Verde, Santa Rita Tlahuapan, Tlaltenango, Huejotzingo, San Miguel Xoxtla, Domingo Arenas, Santa María Atexcac, and the Puebla airport. Explosions on 26 February ejected incandescent tephra 700 m away from the crater onto the N and NE flanks. Ashfall was noted in Domingo Arenas, San Martín Texmelucan, and Huejotzingo in the state of Puebla. The international airport in Huejotzingo suspended operations to clean up the ash. Steam, gas, and ash plumes drifted NE.

On 27 February explosions generated ash emissions and ejected incandescent tephra 300 m onto the flanks. Ashfall was reported in Huejotzingo, Domingo Arenas, Tlaltenango, San Andrés Cholula, and Puebla. During an overflight that same day, volcanologists observed dome number 55 which had grown and was filling the bottom of the inner crater. The dome was 250 m in diameter and at least 40 m thick, putting it about 60 m from the bottom of the main crater floor. The volume was an estimated 1.96 million cubic meters. The volcanologists also observed a small explosion that produced a 1.5-km-high ash plume.

Two separate series of explosions were detected on 28 February, and incandescent tephra was ejected 300 m onto the flanks. Steam-and-gas plumes rose from the crater during 1-2 March. Steam, gas, and ash plumes rose as high as 1.5 km on 3 March. Low-amplitude harmonic tremor and explosions were detected. Ash emissions drifted N. Incandescent tephra was ejected 100-300 m onto the N and NE flanks. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during an overflight of Popocatépetl on 17 February volcanologists observed dome number 55, 150 in diameter, at the bottom of the inner crater (formed in July 2013) which was 100 m below the floor of the main crater. Each day during 18-24 February the seismic network recorded between 47 and 166 low-intensity events, accompanied by steam-and-gas emissions that visibly contained minor amounts of ash most days. Incandescence from the crater was noted some nights. On 18 February five explosions generated plumes that rose no more than 1 km and drifted NE. A series of small explosions detected during 0844-1300 was accompanied by periods of harmonic tremor. On 21 February there were 22 small explosions, some of which ejected tephra 200 m onto the NE flank. Another series of small explosions, detected from 2221 on 22 February to 0220 on 23 February, were again accompanied by periods of harmonic tremor. Steam, gas, and ash plumes drifted SW on 24 February. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 10-11 and 11-12 February the seismic network at Popocatépetl recorded 146 and 101 low-intensity events, respectively, accompanied by steam-and-gas emissions that sometimes contained minor amounts of ash. Explosions were also detected, likely from lava-dome growth. On 11 February ashfall was reported in Puebla (~50 km to the E) and in the municipalities of Juan C. Bonilla, Domingo Arenas, Huejotzingo (27 km NE), and at the airport to the E. Intermittent nighttime incandescence from the crater was visible during 11-12 February.

During 13-17 February seismicity indicated ongoing emissions, and incandescence from the crater was noted. A series of explosions between 0650 and 1200 on 15 February generated plumes that rose 1.8 km above the crater and drifted NE. Ash fell in Huejotzingo, Domingo Arenas, Salvador el Verde (30 km NNE), San Felipe Teotlalcingo (26 km NNE) , and Puebla. Explosions continued to be detected; nine were registered during 16-17 February. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 14-20 January seismicity at Popocatépetl indicated continuing emissions of water vapor and gas, which occasionally contained ash. Cloud cover sometimes prevented views of the crater. Incandescence from the crater was visible nightly, although sometimes only in conjunction with emissions. Explosions on 15 January at 0837 and 1652 produced ash plumes that rose at most 1 km and drifted NE. An explosion on 17 January at 0704 produced an ash plume that rose 1.2 km and drifted E. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 7-10 and 13 January seismicity at Popocatépetl indicated continuing emissions of water vapor and gas, which occasionally contained ash; steam-and-gas plumes were visible during 11-12 January. Cloud cover sometimes prevented views of the crater. Incandescence from the crater was visible nightly. Small explosions on 8 January at 1959 and on 9 January at 0149 produced ash plumes that rose 500 and 800 m, respectively. Three ash plumes recorded during 9-10 January rose 500-800 m and drifted E. An explosion on 12 January generated an ash plume that rose 800 m and drifted NE. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 31 December-6 January seismicity at Popocatépetl indicated continuing emissions of water vapor and gas, which occasionally contained ash. Cloud cover sometimes prevented views of the crater. Incandescence from the crater was visible on a few nights. Explosions on 1 January at 1948 and on 3 January at 1228 produced ash plumes that rose 600 m and drifted NE. An explosion at 0714 on 4 January generated an ash plume that rose 1.3 km and drifted E. An ash plume later that day rose 1.5 km and drifted NE. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 24-30 December seismicity at Popocatépetl indicated continuing emissions of water vapor and gas, which occasionally contained ash during 27-30 December. Cloud cover sometimes prevented views of the crater. Incandescence from the crater was visible some nights. During 24-25 December there were 6-7 explosions detected by the network. An explosion on 26 December generated an ash plume that rose 3.5 km above the crater and drifted NE. Six explosions were detected on 27 December; those at 1258 and 2036 produced ash plumes that rose 0.8-1.5 km and drifted NE. An explosion at 2348 generated a plume with low ash content that rose 0.5 km and drifted S. An explosion on 29 December produced an ash plume that rose 2 km and drifted NE. On 30 December there were 10 explosions that all produced ash plumes that rose 1 km and drifted NE; one of the explosions generated an ash plume that rose 2 km. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that the International airport in Puebla temporarily closed on 17 December due to ashfall from a 0446 explosion at Popocatépetl that generated a 2-km-high ash plume. The explosion also ejected incandescent tephra that landed 700 m down the N flank. Three more explosions were detected that day. During 18-23 December seismicity indicated continuing emissions of water vapor, gas, and frequent ash. Incandescence from the crater was visible each night. Three explosions occurred on 18 December; the last one generated an ash plume that rose 2 km and drifted NE. Explosions on 19 December generated ash plumes that rose 500-800 m. Explosions ejected incandescent tephra that landed 100-200 m down from the crater on the NE and N flanks. During an overflight volcanologists observed a lava dome at the bottom of the crater. Two explosions were detected during 22-23 December. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 10-16 December seismicity at Popocatépetl indicated continuing emissions of water vapor, gas, and ash. Incandescence from the crater was visible each night. Explosions were detected at 0052 and 0132 on 10 December. Multiple explosions on 16 December generated ash plumes that rose 1 km. An explosion at 0259 ejected incandescent material onto the NE flank, less than 200 m from the crater. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 3-9 December seismicity at Popocatépetl indicated continuing emissions of water vapor and gas, which occasionally contained ash during 6-9 December. Incandescence from the crater was visible each night. On 3 December an ash plume rose 800 m above the crater and drifted WSW. An explosion at 2154 was associated with crater incandescence. An ash plume rose 600 m, and explosions at 1056 and 2121 produced ash plumes that rose 800 m and drifted SW. Two ash plumes rose 400-600 m and drifted SW the next day. Explosions detected at 0544 and 0608 on 6 December produced plumes with low ash content that rose 800 m. At 1508 a small rockslide on the N flank was recorded by a webcam. An explosion at 0431 on 8 December ejected incandescent tephra onto the flanks and generated an ash plume that rose 3 km. Slight ashfall was reported in the municipality of Tetela del Volcán (20 km SW). Explosions were detected at 0917 and 0933 on 9 December. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 19-25 November seismicity at Popocatépetl indicated continuing emissions of water vapor and gas. Incandescence from the crater was visible each night. On 21 November a plume with low ash content rose 700 m above the crater and drifted NE. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 12-15 November seismicity at Popocatépetl indicated continuing emissions of water vapor, gas, and small amounts of ash; ash was not observed during 16-18 November. Incandescence from the crater at night was noted. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that a small series of explosions at Popocatépetl, starting at 2003 on 4 November and ending at 0130 on 5 November, produced a continuous plume of gas, steam, and small amounts of ash that rose 1 km and drifted N. The seismic network detected 191 explosions during the period. Incandescent material was periodically ejected onto the N and E flanks, as far as 800 m. Ashfall was reported in Paso de Cortes.

On 6 November a small rockslide on the SW flank was recorded by a webcam and the seismic network. Scientists aboard an overflight observed dome 53, emplaced during 4-5 November; it was an estimated 250 m in diameter and 30 m thick. During 7-11 November seismicity indicated continuing emissions of water vapor, gas, and occasional small amounts of ash. Incandescence from the crater was observed most nights. Explosions were detected during 10-11 November. The first explosion ejected incandescent tephra and generated an ash plume that rose 1.2 km and drifted SE. Others generated plumes that rose as high as 1.2 km and drifted SE and E. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 29 October-4 November seismicity at Popocatépetl indicated continuing emissions of water vapor, gas, and small amounts of ash. Incandescence from the crater was observed most nights. The seismic network detected nine explosions during 29-30 October and two explosions on 31 October; ash plumes rose as high as 1.2 km and drifted SW. Ash plumes rose 1 km and drifted E on 1 November and SW on 3 November. Periodic ejections of incandescent tephra landed 600 m away on the E and N crater flanks on 4 November. Ash plumes rose 1 km. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 22-28 October seismicity at Popocatépetl indicated continuing emissions of water vapor, gas, and small amounts of ash. Incandescence from the crater was observed at night. A small explosion at 0317 on 25 October ejected tephra 100 m onto the S flank. A steam-and-gas plume containing a small amount of ash rose 1.5 km above the crater and drifted SW. Ashfall was reported in Tetela del Volcán (20 km SW). An explosion at 0111 on 26 October ejected tephra 200 m onto the N flank. A steam-and-gas plume with diffuse ash rose 1.1 km and drifted NW. On 28 October an ash plume drifted WSW. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during an overflight of Popocatépetl on 14 October volcanologists observed that the diameter of the inner crater (formed in July 2013) had increased to 350 m. The bottom of the inner crater floor was 100 m below the floor of the main crater, cup-shaped, and covered with tephra. No sign of the 52nd lava dome, emplaced in early August 2014, was visible. Steam emissions originated from a crack in the N wall of the inner crater and ash emission came from the bottom of the crater. During 15-21 October seismicity indicated continuing emissions of water vapor, gas, and occasional small amounts of ash. Cloud cover sometimes prevented visual observations. Incandescence from the crater was observed most nights. Low-intensity explosions were detected on 17 and 19 October. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 24-30 September seismicity at Popocatépetl indicated continuing emissions of water vapor, gas, and occasional small amounts of ash. Incandescence from the crater was observed at night. The Alert Level remained at to Yellow, Phase Two.

During 3-9 September CENAPRED maintained Alert Level Yellow Phase 2 for Popocatepetl. Nighttime incandescence from the crater was visible during this time period. Low-intensity exhalations were observed during 24-hour periods with consecutive daily counts: 15, 1, 5, 17, 30, 12 and 22. On 4 and 5 September, three and four VT earthquakes were detected, respectively, with an average magnitude 1.7. Small pulses of steam and gas drifted WSW during the mornings of 5 and 6 September.

On 7 September, steam and gas emissions were accompanied by 15 minutes of harmonic tremor. At 0315 incandescent tephra was ejected. An ash plume was observed reaching 1 km above the summit and drifting WNW.

CENAPRED reported that during 27 August-2 September explosions were accompanied by steam-and-gas emissions with minor ash and ash plumes that rose 800-3,000 m above Popocatépetl’s crater and drifted W, SW, and WSW. On most nights incandescence was observed, increasing during times with larger emissions. On 1 September partial visibility due to cloud cover was reported. On 29 and 31 August the Washington VAAC reported discrete ash emissions. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 20-26 August steam-and-gas emissions with minor ash rose 100-800 m above Popocatépetl’s crater and drifted NW and W. On 25-26 August the emissions had a low intensity explosive component. On most nights incandescence was observed, increasing with larger emissions. On 23 August slight amounts of ash were reported northwest of the volcano in the communities of Amecameca, Ozumba, and Tlalmanalco. On most days there was only partial visibility due to cloud cover, and on 25-26 August heavy cloud cover was reported. On 24 August the Washington VAAC reported emissions but ash was not detected by satellite. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 13-19 August steam-and-gas emissions with minor ash rose 300-1500 m above Popocatépetl’s crater and drifted NW, W, SW, and N. On most nights incandescence was observed, increasing in intensity with larger emissions. On 14-18 August heavy clouds were reported. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 6-12 August cloudy conditions with intermittent views showed steam-and-gas emissions with minor ash rose above Popocatépetl’s crater. On 7 August there was an explosion and ash was reported in the area of Paso de Cortés. On 9 August there were 5 explosions and ash columns that rose 1 km above the crater. Slight nighttime incandescence was observed. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 24 June-15 July, seismicity at Popocatépetl indicated continuing emissions of water vapor, gas, and occasional small amounts of ash. Cloud cover sometimes prevented visual observations. Slight nighttime incandescence was observed during 25 and 26 June and also during 1-3, 6-8, and 10-15 July. Explosions from the summit were detected an average of 10 times each day, producing plumes with minor ash content that rose 500-2,500 m above the crater and drifted NE and NW. Activity increased in early July; up to 216 explosions (low and moderate intensity) were detected over 24 hours on 9 July. CENAPRED reported harmonic tremor on 2 July (maximum of 80 minutes in 24 hours) and 12 July (minimum of 8 minutes). The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 18-23 June, seismicity at Popocatépetl indicated continuing emissions of water vapor, gas, and occasional small amounts of ash. Cloud cover sometimes prevented visual observations. Slight nighttime incandescence was observed during 18, 19, 20, and 22 June. Explosions from the summit were detected 5-30 times each day and produced plumes with minor ash content that rose 500 m-2,500 m above the crater and drifted NE and NW. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 15-22 April seismicity at Popocatépetl indicated continuing emissions of water vapor, gas, and occasional small amounts of ash. Cloud cover sometimes prevented visual observations. Slight nighttime incandescence was observed. Explosions recorded at 0729 and 1004 on 20 April, at 0758 and 1049 on 21 April, and at 2019 on 22 April produced plumes with minor ash content that rose around 1 km above the crater and drifted NE and NW. Crater incandescence increased when the explosions occurred. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 8-15 April seismicity at Popocatépetl indicated continuing emissions of water vapor, gas, and occasional small amounts of ash. Cloud cover sometimes prevented visual observations. On 9 April some of the larger emissions were accompanied by crater incandescence. A steam-and-gas plume containing small amounts of ash rose 1-1.1 km above the crater and drifted E. Explosions on 10, 12, and 13 April ejected incandescent material 100 m from the crater; ejecta on 12 April landed on the E flank. Nighttime incandescence was visible during 10-15 April. The Alert Level remained at to Yellow, Phase Two.

CENEPRED reported that incandescence from Popocatépetl’s crater was visible at night during 5-11 March, and steam-and gas emissions visible during the day drifted E, NE, and NW. An explosion at 0334 on 6 March ejected material 600 m onto the flanks. The Alert Level remained at Yellow, Phase Two.

On 26 February CENAPRED reported that, with support from the Navy, scientists aboard an overflight of Popocatépetl observed that lava dome 48 had been destroyed, leaving a funnel-shaped cavity about 80 m deep. A new dome 20-30 m wide was at the bottom of the cavity. On 27 February activity decreased considerably. During 27 February-3 March gas-and-steam plumes were observed drifting E, ESE, W, and NE. On 2 March an ash plume rose more than 2 km above the crater and drifted NE. On 4 March at 0552 an explosion ejected incandescent tephra 700 m onto the NE flank and produced an ash plume that rose 1.2 km. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that at 0944 on 19 February a gas-and-vapor plume containing moderate amounts of ash rose 1.5 km above Popocatépetl's crater and drifted NE and SW. At 2338 a gas-and-vapor plume containing small amounts of ash rose 1 km and drifted E. A small explosion at 0409 on 21 February ejected incandescent tephra which mostly fell back inside the crater, and produced an ash plume that rose 2 km. An explosion at 1233 ejected incandescent tephra 600 m from the crater rim. An ash plume rose 4 km and drifted NE. Another explosion at 1541 produced an ash plume with lower ash content that rose 2 km and also drifted NE. At 0312 on 22 February an explosion generated an ash plume that rose 2 km and drifted SE. Explosions at 0615 and 0619 ejected incandescent tephra and produced ash plumes that rose 1 km and drifted SE. On 25 February steam-and-gas plumes drifting SE were occasionally seen during times of good visibility. The Alert Level remained at Yellow, Phase Two.

During 30 October-5 November, CENAPRED maintained Alert Level Yellow, Phase Two. Explosions were frequently detected, varying from 30 to 97 events per day. Though cloudy conditions obscured the view at times, ash plumes were detected on 30-31 October and 1 November. The ash event on 31 October generated a plume that reached an altitude of 1 km and drifted NW.

An Mc 2.1 volcanic-tectonic (VT) earthquake was recorded on 31 October and 4 November; an Mc 2.3 VT earthquake was also detected on 4 November. The largest VT earthquake during this time period was a magnitude 2.5 that occurred at 1031 on 5 November. Tremor was frequently detected during this reporting period; on 1 November, 3 hours and 21 minutes of high frequency tremor were detected.

CENAPRED reported that during 23-25 October seismicity at Popocatépetl indicated continuing emissions of water vapor, gas, and occasional small amounts of ash; cloud cover sometimes prevented observations of the crater. On 24 October an explosion at 2111 produced an ash plume that rose 1 km and drifted SW. Eight low-intensity explosions on 26 October increased gas and steam emissions and produced slight amounts of ash. Incandescence from the crater was observed overnight during 26-27 October. An explosion was detected on 27 October; cloud cover prevented visual observations. An ash plume rose 1 km and drifted W on 28 October. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 4-10 September seismicity at Popocatépetl indicated continuing emissions of water vapor, gas, and small amounts of ash; cloud cover sometimes prevented observations of the crater. Incandescence from the crater was observed on most nights. Steam, gas, and ash plumes drifted SE on 8 September and NW on 9 September. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 28 August-3 September seismicity at Popocatépetl indicated continuing emissions; cloud cover sometimes prevented observations of the crater. Incandescence from the crater was observed on most nights. On 28 August gas, steam, and ash plumes rose 200-800 m and drifted SW. Gas-and-steam plumes were observed the next day. On 30 August gas and steam plumes that sometimes contained ash rose 1 km above the crater and drifted W. During 1-2 September steam and gas plumes containing minor amounts of ash drifted WSW. Ashfall was reported in Tetela del Volcán (20 km SW) and Ocuituco (24 km SW) on 1 September, and in Ecatzingo (15 km SW) on 2 September. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 14-20 August seismicity at Popocatépetl indicated continuing emissions; cloud cover sometimes prevented observations of the crater. Incandescence from the crater was observed and occasionally intensified with some emissions. On 14 August a period of tremor was accompanied by an ash emission that drifted W. Ashfall was reported in the towns of Ozumba (18 km W), Tepetlixpa (20 km W), Atlautla (17 km W), and Ecatzingo (15 km SW) in the State of México. Later that day an ash plume rose 1 km above the crater and drifted W. Gas-and-steam plumes were observed during 15-16 August. A period of tremor on 17 August was accompanied by an ash plume that rose 1.5 km and drifted WSW. Ash fell in Tetela del Volcán (20 km SW), Ocuituco (24 km SW), Yecapixtla (31 km SW), Tlayacapan (40 km WSW), Cuautla (43 km SW), Ayala (45 km SW), and Cuernavaca (65 km WSW). On 18 August high-frequency, low-amplitude tremor was accompanied by an ash emission that rose 1.2 km and drifted SW. On 19 August minor steam-and-gas emissions drifted W. During 19-20 August emissions likely contained small amounts of ash but cloud cover prevented confirmation. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that during 31 July-6 August seismicity at Popocatépetl indicated continuing gas-and-steam emissions that sometimes contained ash; cloud cover often prevented visual confirmation. Incandescence from the crater was occasionally observed. On 31 July a clear decrease in the size of the water vapor and gas plumes was observed; plumes were pushed by winds down the NW flank and rose only 100 m above the crater rim. An explosion was detected at 2312 on 1 August, but cloud cover prevented confirmation of any ejecta. On 2 August minor amounts of ash fell in the Tepetlixpa, Atlautla, Ecatzingo, and Ozumba municipalities of Mexico State. On 4 August emissions of gas, steam, and ash drifted NW. During 5-6 August a few observed plumes rose 1-2 km and drifted WNW, W, and WSW. The Alert Level remained at to Yellow, Phase Two.

CENAPRED reported that on 23 July the Coordinación Nacional de Protección Civil (CNPC) of the Secretaría de Gobernación (SEGOB), CENAPRED, and a Scientific Advisory Committee lowered the Alert Level at Popocatépetl to Yellow, Phase Two. Access to the crater within a 12-km radius was prohibited.

During 24-30 July seismicity indicated continuing gas-and-steam emissions that sometimes contained ash; cloud cover often prevented visual confirmation. During 24-27 July often continuous plumes rose 200 m above the crater and drifted W, NW, and SW. Incandescence from the crater was observed most nights. Ash in the emissions was observed during 26-27 July. At 1237 and 1917 on 28 July, and 0733 on 29 July, ash plumes rose as high as 2 km and drifted W.

CENAPRED reported that during 17-23 July seismicity at Popocatépetl indicated continuing gas-and-steam emissions that sometimes contained ash; cloud cover often prevented visual confirmation. Incandescence from the crater was occasionally observed. On 17 July an explosion was detected at 1516. During a period of clear weather on 19 July observers noted steam-and-gas plumes drifting W. An explosion at 1533 generated a steam, gas, and ash plume that rose 700 m above the crater and drifted NW. Another explosion was detected at 2257. On 20 July steam-and-gas plumes rose 1 km and drifted SW; steam, gas, and ash emissions rose 1.2 km and drifted WSW. Steam-and-gas plumes were bluish on 21 July; the plumes rose 500 m and drifted NW. An explosion at 0343 on 23 July generated an ash plume that rose 1.1 km and drifted NW. The Alert Level remained at to Yellow, Phase Three.

CENAPRED reported that scientists aboard an overflight of Popocatépetl on 10 July confirmed the presence of a new lava dome that was 250 m wide and 20 m thick. During 10-16 July seismicity indicated continuing gas-and-steam emissions that sometimes contained ash; cloud cover often prevented visual confirmation. Incandescence from the crater was observed at night during 9-11 July. Plumes with small amounts of ash were observed at 1556 and 1736 on 10 July, and an explosion was detected at 2259. Medium-sized explosions at 1949 on 11 July, and at 0137 and 0300 on 12 July, ejected incandescent tephra 2 km onto the E flank and 1 km onto the N flank. According to a news article, on 12 July a flight into and out of México City’s (65 km NW) international airport was canceled and operations at a small airport in Puebla (~50 km to the E) were suspended.

Early on 13 July a gas-and-ash plume was observed drifting NE. During 13-14 July steam, gas, and ash emissions rose from the SE part of the crater, some incandescence from the crater was observed, and a dense steam-and-gas plume was noted. On 15 July a plume of steam, gas, and ash rose 1 km and drifted W. During an overflight later that day scientists observed a 200-m-wide and 20-to-30 m deep crater where the lava dome had been; explosions during the previous few days had destroyed the dome. At 0036 on 16 July a steam-and-gas plume containing minor amounts of ash rose from the crater. Incandescence emanating from the crater was also observed early that day. The Alert Level remained at Yellow, Phase Three.

CENAPRED reported that on 3 July the seismic network detected 84 emissions from Popocatépetl. Diffuse ash plumes at 0705 and 0825 rose almost 2 km above the crater. Starting at 1742 tremor was accompanied by persistent emissions of gas and ash that rose 3.5 km. Incandescent tephra was ejected short distances onto the N and E flanks. During 3-4 July tremor and 99 emissions were detected, and incandescence from the crater was observed. Steam-and-gas plumes continued to rise above the crater and incandescent tephra was ejected onto the N and E flanks. According to news articles, multiple airlines canceled 47 flights to and from the México City (65 km NW) and Toluca (105 km WNW) airports on 4 July. Flights resumed later that day. Ash fell in areas as far as México City (70 km NW).

Gas, steam, and ash plumes drifted NW on 5 July, and almost continuous tremor was recorded. CENAPRED staff, with support of the Ministry of the Navy of México, conducted an overflight and observed continuously ejected incandescent tephra deposited at most 1.5 km away on almost all flanks, and an ash plume that rose 2 km. Cloud cover often obscured visual observations. A news article stated that four airlines canceled a total of 17 flights.

On 6 July low-frequency, high-amplitude tremor was accompanied by gas, steam, and ash emissions that rose 2 km and drifted NW. At 1330 the low-frequency tremor amplitude decreased, followed by diminishing emissions of gas and ash which drifted NW. The National Coordination of Civil Protection (CNPC) of the Ministry of Interior (SEGOB), CENAPRED, and Scientific Advisory Committee raised the Alert Level to Yellow, Phase Three. The public was reminded not to approach the crater within a 12-km radius. Later that day gas-and-ash plumes rose 3 km and drifted NW. Three explosions were detected, but cloud cover prevented visual confirmation. News articles noted ash again in parts of México City; ash accumulation was much greater in areas closer to the volcano.

During 7-9 July tremor was accompanied by persistent emissions of steam, gas, and small amounts of ash that drifted WSW and NW; cloud cover continued to hinder visual observations. Three explosions increased gas-and-ash emissions. Incandescence and ejected incandescent tephra were sometimes observed. During an overflight on 7 July, scientists observed that a new lava dome, 250 m in diameter, had recently formed in the crater. Explosions on 8 July generated ash plumes that rose 1 km and drifted NW, and explosions on 9 July generated an ash plume that rose 1.5 km and drifted SW.

CENAPRED reported that at 1448 on 18 June an explosion from Popocatépetl generated an ash plume that rose 2 km above the crater and drifted NW, and ejected incandescent tephra 100 m from the crater. During 19-25 June seismicity indicated gas-and-steam emissions that sometimes contained small amounts of ash; cloud cover often prevented visual confirmation although plumes were observed most days. Incandescence from the crater was occasionally observed and sometimes increased with accompanying emissions. During 23-24 June a water vapor, gas, and ash plume rose 800 m and drifted NW. On 24 June ashfall was reported in Amecameca (20 km NW). The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 12-18 June seismicity at Popocatépetl indicated continuing gas-and-steam emissions that sometimes contained small amounts of ash; cloud cover often prevented visual confirmation. During 12-13 June a total of about 45 minutes of low-amplitude harmonic and high frequency tremors were detected. An explosion at 1716 on 14 June produced an ash plume that rose 1.5 km above the crater. Another explosion at 1727 produced an ash plume that rose almost 3 km. The next day an explosion at 0716 generated an ash plume that rose 2 km. Explosions were also detected at 1610 and 1813.

CENAPRED reported that during 5-11 June seismicity at Popocatépetl indicated continuing gas-and-steam emissions that sometimes contained small amounts of ash; cloud cover often prevented visual confirmation. Incandescence from the crater was observed some nights; during 8-9 June incandescence increased with accompanying emissions. On most days steam-and-gas plumes were observed drifting SW and SSW. On 7 June the Alert Level was lowered to Yellow, Phase Two. An explosion on 8 June generated an ash plume that rose 1 km above the crater and drifted SW. On 9 June ash plumes rose 0.6-2.5 km and drifted SE and E.

CENAPRED reported that during 29 May-4 June seismicity at Popocatépetl indicated continuing gas-and-steam emissions that contained variable amounts of ash; cloud cover often prevented visual confirmation. Incandescence from the crater was often observed at night. On most days steam-and-gas plumes were observed drifting SW and SSW. Some periods of high-frequency and low-amplitude tremor were detected on 31 May and 1 June. During the early morning on 3 June a continuous plume of steam and ash was observed drifting SW. Later that day an ash plume rose 1 km on 3 June. The Alert Level remained at Yellow, Phase Three.

CENAPRED reported that during 22-28 May seismicity at Popocatépetl indicated continuing gas-and-steam emissions that contained variable amounts of ash; cloud cover occasionally prevented observations, especially during 26-27 May. Incandescence from the crater was often observed at night.

On 22 May an ash plume rose 2 km above the crater and drifted NE. Periods of tremor were accompanied by emissions of steam, gas, and sometimes ash. Two plumes rose 1.3 km and drifted W. Overnight incandescent tephra was ejected 300 m above the crater and rolled down the flanks. Tremor amplitude increased on 23 May, and ash emissions drifted SE, S, and SW. An explosion at 0254 ejected large fragments that landed 1.5 km away from the crater. At 1240 an explosion generated a gas-and-ash plume that rose 2.5 km. Later that day tremor decreased; periods of tremor continued to be detected through 27 May, accompanied by emissions of steam, gas and variable amounts of ash that rose 500-900 m and drifted SW.

On 25 May incandescent tephra were ejected onto the highest parts of the N and NE flanks, and a gas-and-ash plume rose 2 km. An explosion at 0547 ejected incandescent tephra 1.5 km onto the NNE flank. An explosion at 1040 on 26 May generated an ash plume that rose 2 km. A small explosion was detected at 1228. On 28 May an explosion at 0503 produced an ash plume that rose more than 2 km and drifted SW, and ejected incandescent tephra 1.5 km onto the NE flank. The Alert Level remained at Yellow, Phase Three.

CENAPRED reported that during 15-21 May seismicity at Popocatépetl indicated continuing gas-and-steam emissions that contained variable amounts of ash; the plumes were sometimes visually confirmed although cloud cover often prevented observations. Incandescence from the crater was often observed at night.

At 0956 on 14 May an explosion generated an ash plume that rose 3 km above the crater and drifted NE, and ejected tephra onto the NE flank at a distance of 600 m. Volcanologists aboard an overflight observed a lava dome 350 m in diameter and 50 m thick, that had slightly deflated after the earlier explosion. An explosion at 0146 on 15 May again generated an ash plume that rose 3 km above the crater, and ejected incandescent tephra onto the flanks at a maximum distance of 1.5 km. At 1804 an explosion produced an ash plume that rose at least 3.5 km and drifted N.

On 16 May gas-and-ash plumes rose 2 km and drifted NE. Minor ashfall was reported in Paso de Cortés, 7 km N. Incandescent tephra was ejected onto the N and NE flanks at a maximum distance of 400 m. The ejections corresponded with several periods of high-frequency, low-amplitude tremor detected between 2020 and 2308, and a swarm that began at 0011 on 17 May. At 2214 an intense explosion ejected incandescent tephra 1.5 km from the crater, and generated an ash plume that rose over 3 km and drifted NE.

At 0028 on 17 May another strong explosion ejected incandescent tephra 1.5 km from the crater, and generated an ash plume that rose over 4 km and drifted NE. Later that day plumes of vapor and gas rose 1 km and drifted SW. During an overflight on 18 May volcanologists observed a crater 200 m wide and 40 m deep in the dome’s surface; the material was likely excavated by the explosions during 14 and 16-17 May. The rest of the dome was covered with rock fragments. Gas-and-ash plumes rose 500 m and drifted SW.

During 19-20 May gas-and-ash plumes drifted E and SW and incandescent tephra was deposited mainly on the NE flanks 400 m away, although most ejected fragments fell back inside the crater. On 21 May steam-and-gas plumes rose a few meters then drifted SSE.

CENAPRED reported that an episode of high-amplitude spasmodic tremor detected at Popocatépetl began between 1928 on 7 May and 0159 on 8 May. The seismic increase was accompanied by an ash plume that rose 3 km above the crater and drifted SE, producing ashfall in San Pedro Benito Juarez (10-12 km SE), San Juan Tianguismanalco (22 km SE), Atlixco (23 km SE), and in some areas of Puebla (~50 km to the E). Incandescent tephra ejected from the crater landed 500 m away on the NE flank. On 8 May an explosion produced an ash plume that drifted SE. Incandescence from the crater was observed at night. The next day gas-and-steam plumes drifted SE.

On 10 May steam, gas, and ash plumes were detected; one of two explosions produced an ash plume that drifted E. A series of ash emissions and periods of harmonic tremor occurred between 1142 and 1443; cloud cover prevented clear views of the ash plumes. On 11 May steam, gas, and ash plumes were again detected. An explosion produced an ash plume that rose 1 km and drifted NE, and ejected incandescent tephra 500 m down the NE flank. Ash possibly fell in villages downwind. Gas-and-ash plumes rose 0.1-2 km and drifted ENE and NE. During 11-12 May periods of spasmodic and harmonic tremor were detected, and activity increased overall.

On 12 May CENAPRED noted that there had been an increase in activity during the previous two weeks, and another intensification that day prompting the Alert Level to be raised to Yellow, Phase Three. Access to the crater within a 12-km radius was prohibited. Stream-and-gas plumes with small amounts of ash rose from the carter. Sporadic ejections of incandescent tephra fell back into the crater and onto the NNE flank, 300 m from the crater rim. On 13 May steam-and-gas plumes were observed rising from the crater during periods of good visibility. On 14 May an explosive event generated an ash plume that rose 3 km and ejected incandescent tephra that landed 600 m away on the NE flank. Cloud cover again obscured summit views. Seismicity remained elevated.

CENAPRED reported that during 10-16 April seismicity at Popocatépetl indicated continuing gas-and-steam emissions that sometimes contained ash. Incandescence from the crater was observed at night and sometimes increased in conjunction with emissions. On 10 April gas-and-steam plumes rose 800 m above the crater and drifted ESE, and ash plumes rose 900 m and also drifted ESE. During 11-13 April gas-and-ash plumes rose 500 m and drifted NE. An explosion on 13 April produced a steam-and-ash plume that rose 400 m and drifted NE. Ashfall was reported in the towns of San Nicolas de los Ranchos (15 km ENE) and Huejotzingo (27 km NE), and in the northern part of Puebla (40 km E). On 14 April a period of tremor was accompanied by continuous emissions of dense steam-and-gas plumes with small amounts of ash that rose as high as 1 km and drifted NE. The next day ash plumes rose 1.5 km above the crater, and incandescent tephra ejected from the crater landed 400 m away on the NE flank. On 16 April gas-and-steam plumes rose 1 km and drifted NE. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 26 March-1 April seismicity at Popocatépetl indicated continuing gas-and-steam emissions that contained ash. Although views of the volcano were often obscured by cloud cover, gas-and-ash plumes were observed daily. Incandescence from the crater was observed at night and sometimes increased in conjunction with emissions. On 26 March incandescent fragments ejected as far as 1 km from the crater landed on the NNE flanks. An explosion produced an ash plume that rose 1.5 km above the crater. On 29 March incandescent fragments ejected 700 m from the crater landed on the N and NE flanks. On 31 March ash emissions were observed continuously for about an hour. Ash plumes rose over 2 km and drifted E. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 19-26 March seismicity at Popocatépetl indicated continuing gas-and-steam emissions that contained ash on most days. Incandescence from the crater was observed at night and increased in conjunction with emissions. During 19-20 March incandescent fragments were ejected 50 m from the crater and landed on the E flank. An explosion on 24 March ejected incandescent fragments 500 m away from the crater that again landed on the E flank. An ash plume rose 1.5 km and drifted E, causing ashfall in villages downwind. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 5-12 March seismicity at Popocatépetl indicated continuing gas-and-steam emissions that contained ash on most days. On 7 March seismicity increased, and incandescent tephra was ejected up to 1 km above the crater and fell on the NE flanks. An eruption plume rose 1.5 km above the crater; ashfall was reported in Puebla (50 km to the E). Activity decreased later that day. On 10 March incandescent tephra was ejected 100 m above the crater and again fell on the NE flanks, 400 m away from the crater. The next day incandescent tephra ejected from the crater fell back into the crater. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 19-26 February seismicity at Popocatépetl indicated continuing gas-and-steam emissions sometimes containing ash. Incandescence from the crater was observed at night. On 19 February an ash plume rose 1.5 km above the crater and drifted E. The next day gas-and-steam emissions rose 800 m above the rim and a diffuse bluish plume rose 1 km. On 21 February a gas-and-ash plume rose 800 m, and on 22 February a gas-and-ash plume rose 1.5 km. Multiple ash plumes that drifted NE were observed on 23 February. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 13-19 February seismicity at Popocatépetl indicated continuing gas-and-steam emissions. Incandescence from the crater was observed at night. Observers reported that gas-and-steam plumes drifted NE, E, and SE; a plume rose 1.2 km above the crater on 19 February. During 18-19 February the emissions possibly contained ash. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 2-8 January views of Popocatépetl were often obscured by cloud cover. Seismicity indicated continuing gas-and-steam emissions that most days contained minor amounts of ash. Variable incandescence from the crater was observed most nights. During 4-8 January, steam-and-gas plumes rose 300 m above the crater and drifted NE or SE. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 12-18 December seismicity at Popocatépetl indicated continuing gas-and-steam emissions that contained minor amounts of ash. Variable incandescence from the crater was observed most nights. During 13-15 December gas-and-steam plumes rose at most 1 km above the crater and drifted NE, E, and SE. On 17 December ejected incandescent tephra landed 500 m away from the crater on the NE flank. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 4-11 December seismicity at Popocatépetl indicated continuing gas-and-steam emissions that contained minor amounts of ash. Incandescence from the crater was observed at night. Cloud cover often prevented observations; gas-and-steam plumes were observed drifting NE, E, and SSE during periods of clearer weather. Ash plumes observed during 7-8 December rose at most 1 km and drifted NE. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 31 October-6 November seismicity at Popocatépetl indicated continuing gas-and-steam emissions that contained ash. Incandescence from the crater was observed at night. Cloud cover often prevented observations; gas-and-steam plumes were observed drifting E and NE during periods of clearer weather. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 23-30 October seismicity at Popocatépetl indicated continuing gas-and-steam emissions that contained ash. Incandescence from the crater was observed at night during 23-24 and 28-30 October. Incandescent tephra was ejected from the crater on 26 October. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 15-23 October seismicity at Popocatépetl indicated continuing gas-and-steam emissions that occasionally contained variable amounts of ash; the plumes rose 0.5-1 km and drifted NW, W, and SW. Incandescence from the crater was observed at night. During 15-16 and 20 October incandescent tephra was ejected 1 km above the crater and fell back in the crater or on the flanks. On 20 October ashfall was reported in Tetela del Volcan (20 km SW). The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 10-16 October seismicity at Popocatépetl indicated continuing gas-and-steam emissions; during 10-13 the emissions observed drifting W and NW. During 14-16 October ash was present in the gas-and-steam plumes. Plumes rose 1.5 km above the crater and drifted W, WSW, and NW. Incandescence from the crater was observed at night. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 25 September-2 October seismicity at Popocatépetl indicated continuing gas-and-steam emissions; cloud cover often prevented visual observations of the volcano. Incandescence from the crater was sometimes observed at night. On most days gas-and-steam plumes rose at most 1.5 km above the crater and drifted in multiple directions. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 12-18 September seismicity at Popocatépetl indicated continuing gas-and-steam emissions that sometimes contained ash; cloud cover often prevented visual observations of the volcano. Incandescence from the crater was observed most nights. On 13 September gas-and-steam plumes rose 2 km above the crater and drifted NE. Later that day a gas-and-ash plume rose 2 km. Incandescent tephra was ejected 500 m and fell on the NE flanks. On 14 September incandescent tephra fell on the N flank and gas-and-ash plumes rose 1 km. The next day gas-and-ash plumes again rose 1 km. During 16-18 September gas plumes continued to rise 1 km at most and drifted NE, NW, and SW. The Alert Level remained at Yellow, Phase Two.

CENAPRED reported that during 29 August-4 September seismicity at Popocatépetl indicated continuing gas-and-steam emissions that may have contained ash; cloud cover often prevented visual observations of the volcano. Incandescence from the crater was observed at night. Gas plumes, that were sometimes bluish, rose up to 1 km above the crater and drifted NE, W, SW, and WSW. On 1 September the Alert Level was lowered to Yellow, Phase Two.

CENAPRED reported that during 22-28 August seismicity at Popocatépetl indicated continuing gas-and-steam emissions that may have contained ash; cloud cover often prevented visual observations of the volcano. During 22-28 August gas-and-steam plumes rose from the crater, and drifted WSW, W, and WNW during 22-24 August. Incandescence from the crater was observed at night during 23-26 August. Bluish steam-and-gas plumes rose from the crater on 27 August. At 2233 an explosion produced an ash plume and ejected incandescent tephra that fell back into the crater. More robust emissions that rose 500 m were sometimes accompanied by incandescence from the crater. Later a plume rose 1.5 km. The next day bluish steam-and-gas plumes rose 1.2 km. The Alert Level remained at Yellow, Phase Three.

CENAPRED reported that during 15-21 August seismicity at Popocatépetl indicated continuing gas-and-steam emissions that may have contained ash; cloud cover often prevented visual observations of the volcano. Incandescence from the crater was observed at night. A gas-and-ash plume rose from the crater on 15 August, and steam-and-gas plumes rose as high as 1 km and drifted NW the next day. On 17 August seismicity significantly increased. Incandescent tephra ejected from the crater fell onto the flanks and rolled 800 m. On 18 August an ash plume rose 2 km above the crater and drifted E. Ejected incandescent tephra fell onto the SE flank. A small ash emission and ejected incandescent tephra were observed on 19 August. The next day a steam-and-gas plume rose 900 m and drifted NNW. The Alert Level remained at Yellow, Phase Three.

CENAPRED reported that during 8-14 August seismicity at Popocatépetl indicated continuing gas-and-steam emissions that may have contained ash on 8 August; cloud cover prevented observations during most of this period. Incandescence from the crater was periodically observed. Gas-and-steam plumes were observed rising from the crater during 8-9 and 14 August. A small ash emission was observed on 14 August. The Alert Level remained at Yellow, Phase Three.

CENAPRED reported that during 1-7 August seismicity at Popocatépetl indicated continuing gas-and-steam emissions that may have contained ash during 4-6 August; cloud cover prevented observations during most of this period. Incandescence from the crater was periodically observed. Gas-and-steam plumes were observed almost daily rising from the crater as high as 2.5 km above the rim. The plumes drifted SW and NW. On 5 August ash plumes rose 1.5 km and drifted SW. On 6 August ash plumes again rose 1.5 km above the crater, and at 1758 an ash plume rose 4 km. Some explosions ejected incandescent tephra that landed on the flanks. Gas-and-steam plumes drifted SW the next day. The Alert Level remained at Yellow, Phase Three.

CENAPRED reported that during 25-31 July seismicity at Popocatépetl indicated continuing gas-and-ash emissions that sometimes probably contained ash; cloud cover prevented observations during most of this period. Incandescence from the crater was periodically observed. Steam-and-gas plumes were observed daily rising from the crater as high as 1.5 km above the rim. The plumes drifted in multiple directions. The Alert Level remained at Yellow, Phase Three.

CENAPRED reported that during 18-23 July seismicity at Popocatépetl indicated continuing gas-and-ash emissions; cloud cover prevented observations during most of this period. Incandescence from the crater was periodically observed and sometimes increased with accompanying emissions. A steam-and-gas plume rose from the crater on 19 and 24 July, and ash-and-gas plumes were observed on 20 and 22 July. Activity increased for a period during 20-21 July; incandescent tephra was ejected 500 m above the crater and ashfall was reported in Ozumba (18 km W). The Alert Level remained at Yellow, Phase Three.

CENAPRED reported that during 10-17 July seismicity at Popocatépetl indicated continuing gas-and-ash emissions; cloud cover prevented observations during most of this period. Incandescence from the crater was periodically observed and sometimes increased with accompanying emissions. On 11 July a gas-and-steam plume rose 1.5 km above the crater and drifted WSW. On 16 July an ash plume rose 1.2 km and drifted SE. A few minutes later a gas-and-steam plume rose 1.5 km. The Alert Level remained at Yellow, Phase Three.

CENAPRED reported that during 4-10 July seismicity at Popocatépetl indicated continuing gas-and-ash emissions. Incandescence from the crater was often observed at night. On 4 July a gas-and-steam plume drifted W. The next day a steam-and-gas plume that likely contained ash rose 2 km above the crater and drifted WSW. Some emissions were accompanied by increased incandescence from the crater. During 6-10 July steam-and-gas plumes rose from the crater. The Alert Level remained at Yellow, Phase Three.

CENAPRED reported that during 20-25 June meteorological weather conditions often prevented observations of Popocatépetl's crater. Emissions were detected by the seismic network and an occasional steam plume was seen rising from the crater. On 24 June a gas-and-steam plume containing a small amount of ash drifted NE and a dense gas-and-steam plume drifted SW. Incandescence from the crater was occasionally visible at night. The next day emissions sometimes contained small amounts of ash. On 26 June visibility improved. Increased incandescence from the crater was accompanied by gas and very dark ash emissions that rose 0.8-2 km above the crater. The Alert Level remained at Yellow Phase Three.

CENAPRED reported that during 13-19 June meteorological cloud cover often prevented observations of Popocatépetl's crater. Incandescence from the crater was occasionally visible at night. During 13-15 June gas-and-ash plumes that rose above the crater sometimes drifting NW and W. Ejected tephra fell onto the E, N, and W flanks, as far away as 500 m from the crater. The Alert Level remained at Yellow Phase Three.

CENAPRED reported that during 6-12 June gas-and-ash plumes from Popocatépetl rose above the crater, sometimes as high as 2 km, and drifted NNW, E, ESE, SSE, and S. Cloud cover occasionally prevented observations of the crater and plumes. Incandescence from the crater was visible on some nights. Incandescent fragments ejected from the crater fell onto the flanks on 8 and 11 June, and back into the crater on 10 June. The Alert Level remained at Yellow Phase Three.

CENAPRED reported that during 30 May-5 June gas-and-ash plumes from Popocatépetl rose as high as 2 km above the crater and drifted mostly W and SW, but also NW and ENE. Cloud cover occasionally prevented observations of the plumes and crater. Incandescence from the crater was visible on most nights. Incandescent fragments were ejected from the crater on 5 June. The Alert Level remained at Yellow Phase Three.

CENAPRED reported that during 23-29 May gas-and-ash plumes from Popocatépetl rose up to 2 km above the crater and drifted in multiple directions. Cloud cover occasionally prevented observations of the plumes. Ashfall was reported in San Pedro Benito Juarez (10-12 km SE) and Huejotzingo (27 km NE) on 23 May, and in Atlixco (23 km SE) and San Pedro Benito Juarez on 25 May. Incandescent fragments ejected from the crater landed on the flanks during 23-26 May. Incandescence from the crater was visible on 27 May. The Alert Level remained at Yellow Phase Three.

CENAPRED reported that during 8-10 May multiple gas-and-ash plumes rose 1.5 km above Popocatépetl's crater and drifted NE. Incandescent fragments ejected from the crater landed on the N and E flanks as far as 500 m away. According to a news article an airport in Puebla was closed due to ash plumes on 8 and 10 May. Seismicity increased on 11 May. Ash plumes rose 3 km above the crater and drifted NE and incandescent fragments ejected from the crater rolled 1 km down the flanks. Gas-and-ash plumes rose 2.5 km and drifted ENE. On 12 May gas-and-ash plumes rose 4 km above the crater. Incandescent tephra was ejected 2 km above the crater and again rolled 1 km down the flanks. Ashfall was reported in most municipalities within the state of Tlaxcala (50 km NE of the volcano), in addition to smaller towns nearer to the volcano such as Santiago Xalitzintla (15 km NE) and San Nicolás de los Ranchos (16 km ENE). An airport in Puebla was again closed due to ash. During 13-15 May gas-and-ash plumes rose from the crater and drifted NE, and incandescent tephra was ejected from the crater. The Alert Level remained at Yellow Phase Three.

CENAPRED reported that during 2-3 May activity at Popocatépetl increased significantly. Spasmodic tremor was detected along with a dense and continuous plume of gas and ash that drifted W, NW, and NNE. Ash fell in multiple areas, including Amecameca (20 km NW), Atlautla, Ozumba (18 km W), Ecatzingo (15 km SW), Chalco (35 km NW), and some parts of SE México City (70 km NW). On 3 May gas-and-ash plumes rose 1.5 km above the crater and drifted W and NE. Incandescent fragments ejected from the crater landed on the flanks as far as 800 m away. Explosion-generated gas-and-ash plumes the next day rose 2.5 km above the crater and drifted NW. Spasmodic tremor was detected along with a dense and continuous plume of gas and ash that drifted WNW. Later that day gas-and-ash plumes rose 1 km. During 5-6 May gas-and-ash plumes rose 1.5 km above the crater and drifted N, NE, and E. Light ashfall was reported in Atlixco (25 km SE), San Juan Tianguismanalco (22 km SE), Tochimilco (16 km SSE), San Pedro Benito Juárez (10 km SE), and San Nicolás de los Ranchos (16 km ENE). During 5-7 May incandescent fragments ejected from the crater landed on the flanks as far as 500 m away. A gas-and-ash plume drifted ESE on 7 May. The next day activity remained high; seismic events were accompanied by dense and continuous plumes of steam, gas, and ash that drifted mainly ESE. The Alert Level remained at Yellow Phase Three.

CENAPRED reported that gas-and-steam plumes, occasionally containing ash, rose from Popocatépetl during 25-29 April. Gas-and-ash plumes rose 1.5 km above the crater on 25 and 27 April. On 28 April incandescent fragments ejected from the crater landed on E flank as far as 1 km. The next day steam-and-gas plume rose from the crater. On 1 May gas-and-steam plumes, that occasionally contained low amounts of ash, and rose 1 km above the crater. The Alert Level remained at Yellow Phase Three.

CENAPRED reported that multiple gas-and-ash plumes rose from Popocatépetl on 18 April; one of the emissions was accompanied by increased incandescence in the crater. An explosion ejected incandescent fragments that landed on the N and NE flanks as far as 800 m from the crater. The fragments landed on snow and generated small lahars. A dense gas, steam, and ash plume drifted E and SE. On 19 April gas-and-ash plumes rose above the crater and drifted ESE, and incandescent fragments rolled 1 km down the flanks. The next day an episode of spasmodic tremor was accompanied by a dense plume of gas, water vapor, and ash that rose 1.5 km and drifted E. During 21-23 April gas-and-steam emissions that sometimes contained small amounts of ash drifted SE, E, and SW. Seismicity was low during 21-22 April and again increased on 23 April. That same day an ash plume drifted NE and incandescent fragments were ejected W. The Alert Level remained at Yellow Phase Three.

CENAPRED reported that during 11-15 April steam-and-gas emissions from Popocatépetl occasionally contained ash; emissions contained a substantial amount of ash on 12 April. Seismicity increased on 13 April and at 2220 an explosion ejected incandescent blocks that landed on the NE flank as far as 500 m away from the crater rim. A larger explosion at 2236 ejected incandescent blocks that landed even further away on all flanks; an ash plume rose 2 km above the crater and drifted ENE. Ashfall was reported in San Pedro Benito Juarez (10-12 km SE), where the explosion was also heard. On 14 April gas-and-steam plumes that contained small amounts of ash drifted SW. Multiple emissions occurred with increased incandescence from the crater. Ejected incandescent blocks landed back in the crater or on the flanks 500-800 m from the rim. Gas-and-ash plumes drifted ESE. Ashfall was reported in multiple towns, including Puebla (50 km to the E), San Pedro Benito Juarez, Santiago Xalitzintla (15 km NE), Tianguismanalco, and Atlixco (25 km SE).

On 15 April an ash plume rose 1.5 km above the crater and drifted E. Gas-and-ash emissions rose 1 km above the crater on 16 April and were accompanied by ejected incandescent fragments that were deposited on the flanks, especially to the N and NE. Later that day ash plumes rose 2 km above the crater and drifted E. Ashfall was again reported in Puebla. CENAPRED increased the Alert Level at the volcano from Yellow Phase Two to Yellow Phase Three. During 16-17 April incandescence extended 300 m above the crater and gas-and-steam emissions were constant. Gas-and-ash plumes rose from the crater on 17 April.

CENAPRED reported that during 27 March-3 April steam-and-gas emissions rose from Popocatépetl. Emissions contained small amounts of ash on 28 and 30 March and crater incandescence was observed at night during 31 March-3 April.

CENAPRED reported that during 14-20 March steam-and-gas emissions rose from Popocatépetl and incandescence from the crater was observed at night. Emissions contained small amounts of ash on 14 March. On 18 March emissions again contained a small amount of ash and were accompanied by increased incandescence from the crater.

CENAPRED reported that during 1-6 March steam-and-gas emissions rose from Popocatépetl. Emissions contained small amounts of ash on 1 March and crater incandescence was observed at night. During the night on 2 March crater incandescence rose 200-300 m above the crater.

CENAPRED reported that during 15-21 February steam-and-gas emissions rose from Popocatépetl; some of the emissions contained small amounts of ash on 16 and 20-21 February. Crater incandescence was observed during 16 and 19-21 February.

CENAPRED reported that during 8-14 February steam-and-gas emissions rose from Popocatépetl; some of the emissions contained small amounts of ash on 14 February. Crater incandescence was observed during the morning of 8 and 14 February and from the S on 13 February. Clouds prevented views during 9-12 February.

CENAPRED reported that during 1-7 February steam-and-gas emissions rose from Popocatépetl; some of the emissions contained small amounts of ash during 1-4 February. Gas-and-steam plumes rose 200-800 m above the crater during 1-5 February. Crater incandescence was observed at night on 1 and 5 February. Clouds prevented views on 7 February.

CENAPRED reported that during 25-31 January steam-and-gas emissions rose from Popocatépetl; some of the emissions contained moderate amounts of ash on 25 and 29 January. On 25 January an ash plume rose 3 km above the crater and drifted NE. Crater incandescence was observed at night on 29 and 31 January.

CENAPRED reported that during 18-24 January steam-and-gas emissions rose from Popocatépetl, that occasionally contained small amounts of ash during 19-22 January. On 19 January a plume rose 2.5 km above the crater and drifted NE. Crater incandescence was observed at night during 19-21 January.

CENAPRED reported that during 11-17 January steam-and-gas emissions rose from Popocatépetl; the plumes contained small amounts of ash during 13-17 January. Crater incandescence was observed at night during 16-17 January.

CENAPRED reported that during 4-10 January steam-and-gas emissions rose from Popocatépetl; the plumes contained small amounts of ash during 4-5 January and 8-10 January. On 5 January two explosions generated incandescence in the crater.

CENAPRED reported that during 28 December-3 January steam-and-gas emissions rose from Popocatépetl and a gas plume rose to an altitude of 1 km on 28 December. Two small landslides were recorded inside the crater on 2 January. During 2-3 January gas plumes rose to an altitudes of 600-700 m above the crater and drifted NE.

CENAPRED reported that during 21-27 December steam-and-gas emissions rose from Popocatépetl; the plumes contained small amounts of ash during 22-24 December. On 21 December a bluish gas plume was observed and during 22-23 December ash plumes rose 0.9-1 km above the crater. Crater incandescence was observed at night during 26-27 December.

CENAPRED reported that during 14-20 December steam-and-gas emissions rose from Popocatépetl. Those emissions occasionally contained small amounts of ash on 14, 15, and 20 December. On 15 December an ash plume rose 1.2 km above the crater and drifted NE. During 18-19 December crater incandescence was observed early in the morning. An ash plume rose 900 m above the crater on 20 December.

CENAPRED reported that during 7-13 December steam-and-gas emissions rose from Popocatépetl and occasionally contained small amounts of ash. On 7 December ashfall was reported in San Pedro (13.5 km NW). Crater incandescence was observed during 9-11 December and on 9 December incandescence ballistic fragments were observed on the upper slopes of the cone.

Based on reports from CENEPRED and the Mexico City MWO, the Washington VAAC reported an ash plume that drifted 12.9 km W on 7 December.

CENAPRED reported that during 30 November-6 December steam-and-gas emissions rose from Popocatépetl and crater incandescence was observed during most nights and early mornings. Steam-and-gas emissions occasionally contained small amounts of ash on 30 November and 5 December.

Based on information from the Mexico City MWO, the Washington VAAC reported that an ash plume rose to an altitude of 6.7 km (22,000 ft) a.s.l. and drifted NE on 5 December. That same day satellite imagery showed the ash plume had dissipated.

CENAPRED reported that during 22-28 November steam-and-gas emissions rose from Popocatépetl and crater incandescence was observed during most nights and early mornings. Steam-and-gas emissions occasionally contained small amounts of ash on 29 November.

During 16-20 November CENAPRED reported steam-and-gas emissions from Popocatépetl. A series of emissions was detected on 18 November; clouds prevented ground observations and no ashfall was reported. Based on information from the Mexico City MWO however, the Washington VAAC reported that an ash plume rose to an altitude of 7.6 km (25,000 ft) a.s.l. and drifted SW that same day. Satellite imagery showed another ash plume drifting E at an approximate altitude of 9.1 km (30,000 ft) a.s.l. CENAPRED noted that at 1201 on 20 November an explosion produced an ash plume that rose approximately 2 km above the crater and drifted N. The explosion was heard in Amecameca (19 km NW). Steam-and-gas plumes rose from the crater during 21-22 November.

During 21-25 September CENAPRED reported steam-and-gas emissions from Popocatépetl. An explosion on 26 September ejected incandescent fragments and produced an ash plume that rose 2.5 km above the crater. Following the explosion, a series of 11 gas-and-steam plumes containing small amounts of ash rose from the crater.

CENAPRED reported that on 29 August instances of emissions of gas, steam, and some ash from Popocatépetl increased. The next day an ash plume rose 1 km above the crater and drifted WNW, producing ashfall in San Pedro Nexapa (14 km NW) and Amecameca (19 km NW). CENAPRED noted that recent rain in the area may have contributed to the recent increase in activity. During 30-31 August there were 111 plumes of gas, steam, and some ash detected by the network, in addition to periods of harmonic tremor. Signals from detectors near drainages possibly indicated lahars. During 1-4 September the monitoring network registered 4-12 instances daily of emissions of gas, steam, and some ash. Periods of tremor continued to be detected.

CENAPRED reported that, although cloud cover often prevented observations of Popocatépetl during 24-30 August, steam-and-gas emissions were occasionally noted. Based on reports from CENEPRED and the Mexico City MWO, the Washington VAAC reported ash emissions during 28-29 August.

CENAPRED reported that late on 9 August an ash plume from Popocatépetl rose 1 km above the crater and drifted W. During 11-12 August steam-and-gas emissions occasionally contained small amounts of ash.

CENAPRED reported that during 31 May-1 June steam-and-gas emissions from Popocatépetl occasionally contained small amounts of ash. At 0637 on 3 June an ash plume rose 3 km above the crater following seismic tremor. The lower-altitude portion of the plume drifted W (towards the state of Morelos) and the higher-altitude portion of the plume drifted ENE (over Puebla, 40 km E). Within a few hours ashfall was reported in the Morelos state, municipalities of Tetela del Volcán (20 km SW), Zacualpan (31 km SW), Jonacatepec (43 km SW), and Axochiapan (60 km SSW). At 2112 high-frequency, low-amplitude tremor was detected that was followed by an ash plume at 2116 that rose 1 km above the crater and drifted W. By 2130 activity had returned to normal levels. On 4 June an ash plume rose 1 km above the crater and drifted SSW at lower altitudes and NE at higher altitudes.

CENAPRED reported that on 19 May steam-and-gas emissions from Popocatépetl occasionally contained small amounts of ash. The Washington VAAC noted that satellite imagery on 22 May showed a rapidly dissipating area of ash about 650 km SW.

Based on information from the Mexico City MWO, analyses of satellite imagery, and web-camera views, the Washington VAAC reported that on 26 March an ash-and-gas plume from Popocatépetl drifted ESE and SE. A subsequent notice that day stated that the ash had dissipated approximately 140 km SE. On 27 March a small ash plume drifted almost 150 m E.

CENAPRED reported that during 2-4 and 6 March steam-and-gas emissions from Popocatépetl occasionally contained small amounts of ash. The Washington VAAC noted that a gas-and-ash plume drifting 130 km SE at an altitude of 9.1 km (30,000 ft) a.s.l. on 3 March.

CENAPRED noted that steam-and-gas emissions rose from Popocatépetl during 26 January-1 February. An explosion at 2056 on 31 January ejected incandescent fragments as far as 500 m down the E flank and produced an ash plume that rose 2 km and drifted E. The Washington VAAC noted that the ash plume was observed in satellite imagery on 1 February drifting more than 275 km NE.

CENAPRED noted that steam-and-gas emissions rose from Popocatépetl during 1-7 December. Based on a SIGMET notice, the Washington VAAC reported that a small and brief emission of gas and ash on 5 December was too small or diffuse to be detected in satellite images.

Based on information from the Mexico City MWO, the Washington VAAC reported a small and brief emission from Popocatépetl on 12 November. CENAPRED reported four steam-and-gas emissions during 12-13 November.

CENAPRED reported that during 9-11 June steam-and-gas emissions from Popocatépetl occasionally contained small amounts of ash. On 10 June an ash-and-steam plume rose 800 m above the crater. The next day an ash-and-gas plume rose 1 km above the crater. Later that day, another ash-and-gas plume rose 600 m.

CENAPRED reported that during 2-3 and 7-8 June steam-and-gas emissions from Popocatépetl occasionally contained small amounts of ash. The seismic network detected a few periods of harmonic tremor. Steam-and-gas emissions continued during 4-7 June. On 8 June a moderate explosion generated an ash plume that rose to an altitude of 8.4 km (27,600 ft) a.s.l.

CENAPRED reported that during 18-20 November steam-and-gas plumes from Popocatépetl sometimes contained ash. A small explosion detected by the seismic network on 21 November was accompanied by an ash plume that rose to an altitude of 8.9 km (29,200 ft) a.s.l. The ash plume drifted E and caused ashfall in Atlixco (23 km SE), Huejotzingo (27 km NE), and areas in the state of Tlaxcala.

CENAPRED reported that during 26-27 August emissions of steam and gas from Popocatépetl sometimes contained slight amounts of ash. Based on information from the Mexico City MWO, the Washington VAAC reported that on 27 August an ash plume rose to an altitude of 6.1 km (20,000 ft) a.s.l. and drifted SW.

Based on information from the Mexico City MWO and analyses of satellite imagery, the Washington VAAC reported that on 20 August an ash plume from Popocatépetl rose to an altitude of 8.2 km (27,000 ft) a.s.l. and drifted 13 km W. An ash plume that drifted more than 35 km NNE was seen in satellite imagery on 25 August. CENAPRED reported that during 20-25 August emissions of steam and gas contained slight amounts of ash.

CENAPRED reported that emissions of steam and gas from Popocatépetl were visible during 18-24 March; the plumes contained slight amounts of ash during 20-21 and 23 March. An explosion on 23 March ejected incandescent fragments that landed near the crater. Based on information from CENEPRED, the Washington VAAC reported that a minor emission on 23 March produced a plume that rose to an altitude of 6.7 km (22,000 ft) a.s.l. A small ash plume was seen on satellite imagery drifting SE.

CENAPRED reported that emissions of steam and gas from Popocatépetl were visible during 11-17 February; the plumes occasionally contained slight amounts of ash. On 13 February, a plume with low ash content rose to an altitude of 7.2 km (23,600 ft) a.s.l. and drifted NE at 2230; 95 minutes of increased seismicity followed.

CENAPRED reported that emissions of steam and gas from Popocatépetl were visible during 4-10 February; the plumes occasionally contained slight amounts of ash. On 6 February, an ash plume rose 800 m above the crater at 0839, and was followed by 75 minutes of increased seismicity.

Based on analysis of satellite imagery, information from the Mexico City MWO, and views from the web camera operated by CENAPRED, the Washington VAAC reported that on 21 January an ash plume from Popocatépetl rose to an altitude of 7 km (23,000 ft) a.s.l. and drifted E and NE. A thermal anomaly was also detected. CENAPRED reported that during 21-27 January emissions of steam and gas were noted, and occasionally contained slight amounts of ash during 22-25 January. On 22 January, a small explosion produced an ash plume that rose to an altitude of 7.4 km (24,300 ft) a.s.l.

CENAPRED reported that emissions of steam and gas from Popocatépetl were visible during 12-18 November. Based on analysis of satellite imagery and information from the Mexico City MWO, the Washington VAAC reported that on 17 November an ash plume rose to an altitude of 6.1 km (20,000 ft) a.s.l. and drifted NW and WSW. A thermal anomaly was also detected. According to CENAPRED, the emissions contained slight amounts of ash on 18 November.

CENAPRED reported that emissions of steam and gas from Popocatépetl were visible during 12-18 March. The plumes occasionally contained slight amounts of ash. Ash plumes on 17 March rose to altitudes of 7.4-7.9 km (24,300-26,000 ft) a.s.l. and drifted NE.

CENAPRED reported that emissions of steam and gas from Popocatépetl were visible during 5-11 March. The plumes occasionally contained slight amounts of ash. An explosion on 8 March resulted in ash plumes that rose to an altitude of 6.4 km (21,000 ft) a.s.l. and drifted NE. An ash plume was spotted the next day that rose to the same altitude and also drifted NE.

CENAPRED reported that emissions of steam and gas from Popocatépetl were visible during 20-26 February. The plumes occasionally contained slight amounts of ash. Explosions on 21 and 22 February resulted in ash plumes that rose to an altitude of 7.4 km (24,300 ft) a.s.l. and drifted NE. Incandescent fragments were ejected from the crater and fell on the flanks. On 22 February, the ejected fragments fell on the SE flank.

CENAPRED reported that emissions of steam and gas from Popocatépetl were visible during 12-19 February. The plumes occasionally contained slight amounts of ash. An explosion on 12 February resulted in an ash plume that rose to an altitude of 7 km (23,000 ft) a.s.l. and ejected fragments that fell in the crater. On 14 February, an ash plume rose to an altitude of 8.4 km (27,600 ft) a.s.l. and drifted NE.

CENAPRED reported that emissions of steam and gas from Popocatépetl were visible during 6-12 February. The plumes occasionally contained slight amounts of ash. On 8 February, ash emissions were occasionally accompanied by explosions and propelled incandescent fragments that landed in the vicinity of the crater. Two explosive events on 11 February resulted in ashfall in the town of Huejotzingo, Puebla.

CENAPRED reported that emissions of steam and gas from Popocatépetl were visible during 30 January-5 February. The plumes occasionally contained slight amounts of ash. On 4 February, ash emissions were accompanied by an explosion that propelled incandescent fragments 300 m from the crater.

CENAPRED reported that emissions of steam and gas from Popocatépetl were visible during 23-29 January. The plumes occasionally contained slight amounts of ash. On 28 January, an ash plume rose to an altitude of 8.6 km (28,200 ft) a.s.l. and drifted NW.

CENAPRED reported that emissions of steam and gas from Popocatépetl were visible during 9-14 January. The plumes occasionally contained slight amounts of ash during 9-12 January. On 14 January, high-frequency seismic tremor was followed by an explosion that produced ash emissions and propelled fragments from the crater.

CENAPRED reported that emissions of steam-and-gas from Popocatépetl were visible during 2-8 January. On 5 January, a steam-and-gas plume with low ash content was reported. An altitude of the plume was unknown due to cloud cover. About two hours later, CENAPRED received reports of slight ashfall in Paso de Cortés, about 7.5 km N of the summit.

Based on information from the Mexico City MWO and observations of satellite imagery, the Washington VAAC reported that an ash plume rose to an altitude of 7.3 km (24,000 ft) a.s.l. and drifted E, NE, and SE on 5 January.

CENAPRED reported that emissions of steam-and-gas from Popocatépetl were visible during 30 December-1 January. On 31 December, a 13-minute-long high-frequency tremor event was followed by an emission of a plume with low ash content. The plume rose to an altitude of 7.4 km (24,300 ft) a.s.l. and drifted E and SE.

CENEPRED reported that multiple gas-and-steam plumes from Popocatépetl were observed during 28 November-4 December. On 1 December, high frequency seismic tremor was accompanied by an ash plume that rose to an altitude of 7.4 km (24,300 ft) a.s.l. and drifted N, and then NE. Ashfall was reported in areas downwind. Based on observations of satellite imagery, reports from the Mexico City MWO, and the web camera operated by CENEPRED, the Washington VAAC reported that the ash plume rose to an altitude of 9.1 km (30,000 ft) a.s.l.

According to the Washington VAAC, CENAPRED reported that on 28 July an eruption plume from Popocatépetl with minor ash content was visible on a web camera. The plume rose to an altitude of 7 km (23,000 ft) a.s.l. and drifted WSW. A pilot reported an ash plume on 3 August. Ash was not identified on satellite imagery on either day.

Based on information from the Mexico City MWO and a web camera operated by CENEPRED, the Washington VAAC reported that an ash plume from Popocatépetl rose to an altitude of 6.4 km (21,000 ft) a.s.l. and drifted SSW on 28 June.

According to the Washington VAAC, a puff with little ash content emitted from Popocatépetl was reported from the MWO and visible from the camera operated by CENEPRED on 14 February. A very diffuse plume was seen drifting to the E on satellite imagery. Base on an aerial photograph taken on 24 January, CENEPRED reported that the lava-dome dimensions have slightly increased since 24 November 2006.

According to the Washington VAAC, puffs with little ash content emitted from Popocatépetl were reported from the MWO and visible from the camera operated by CENEPRED during 7-8 January. The resulting eruption clouds drifted NE and N. A hotspot at the summit was detected in satellite imagery.

During 21-27 December, several emissions of gas, steam, and small amounts of ash occurred at Popocatépetl. A moderate explosion on 25 December produced an ash plume to ~8.5 km (28,000 ft) a.s.l. that drifted ENE.

Based on information from the Mexico City MWO, pilot reports, and satellite imagery, the Washington VAAC reported that an eruption plume from Popocatépetl rose to an altitude of 7.9 km (26,000 ft) a.s.l. and drifted E on 20 December. A hotspot was also visible on satellite imagery.

According to the Washington VAAC, emissions of gas, steam, and possibly ash from Popocatépetl were visible from the camera operated by CENEPRED during 4-5 September. The resulting eruption cloud drifted W and did not rise high above the summit. Incandescence was periodically observed at the summit.

According to the Washington VAAC, the Meteorological Watch Office reported emissions from Popocatépetl on 3 August. Ash was not visible on satellite imagery. A bright glow was visible with the camera operated by CENEPRED.

According to the Washington VAAC, ash plumes from Popocatépetl were visible on satellite imagery on 25 and 27 July. These were reported by the Mexico City Meteorological Watch Office to reach altitudes of 9.8 km (32,000 ft.) a.s.l. and drift WSW.

The Washington VAAC reported that ash from Popocatépetl was visible on satellite imagery on 3 April at 0245 at a height of ~7.3 km (~24,000 ft) a.s.l., drifting S. The Mexico City MWO indicated that there was an eruption around this time that emitted small amounts of ash.

During 1-7 February, several small-to-moderate emissions of steam, gas, and small amounts of ash occurred at Popocatépetl. On the 4th, an explosion produced a plume that rose to ~6.7 km (22,000 ft) a.s.l.

During 24-30 January, several emissions of gas, steam, and small amounts of ash occurred at Popocatépetl. A moderate explosion on 26 December at 0957 produced an ash plume to ~3 km (9,850 ft) a.s.l. that drifted NE.

A small explosion occurred at Popocatépetl on 6 January around 0042. According to the Washington VAAC, the resultant ash plume was visible on satellite imagery at a height of ~5.8 km (19,000 ft a.s.l.), extending NE. CENAPRED reported that after the explosion overall activity decreased to previous levels.

During 14-20 December, several emissions of gas, steam, and small amounts of ash occurred at Popocatépetl. Small explosions produced ash plumes to a height of ~2.5 km above the summit (or 26,000 ft a.s.l.).

On 1 December at 0653, a moderate explosion at Popocatépetl produced an ash plume to a height of ~5 km above the volcano's summit (or 34,200 ft a.s.l.). An intense episode lasted ~2 minutes, and was followed by high-frequency tremor for 30 minutes. Ash drifted NE and fell in Amecameca, ~20 km NW of the volcano. An explosion on 1 December at 0920 produced an ash plume to ~2.5 km above the summit (or 26,000 ft a.s.l.) that also drifted NE. After this explosion, there was an increase in the number of small explosions, some of which involved small amounts of ash. After an explosion on 4 December, ash fell in the states of Tlaxcala (50 km NE of the volcano) and Puebla (60 km E of the volcano) and the volcanic activity returned to previous lower levels. As of 12 December, only emissions of steam and gas occurred.

Based on information from the Mexico City MWO, the Washington VAAC reported that an emission from Popocatépetl occurred on 23 October around 1700. The resultant ash cloud rose to a height of ~6.4 km (21,000 ft) a.s.l. and drifted SE.

Several low-intensity emissions occurred at Popocatépetl during 21-25 July. On 21 July at 0906, an emission produced a plume to a height of ~2 km above the volcano's crater (~24,400 ft a.s.l.). Based on information from the Mexico City MWO, the Washington VAAC reported that ash visible on the web video camera rose to ~2.8 km above the crater (27,000 ft a.s.l.). The Alert Level at Popocatépetl remained at Yellow Phase I.

On 14 July at 1005 a moderate-sized emission from Popocatépetl produced an ash plume to a height of ~2 km above the volcano's crater (~24,400 ft a.s.l.). The emission was preceded by high-frequency tremor. During the remainder of 14-18 July, there were small emissions from the volcano.

During 22-27 June, Popocatépetl volcano had several steam explosions. On 22 June, there was a volcano-tectonic micro-earthquake of magnitude 2.0, located 500 m NW of the crater at a depth of 4.6 km. On 23 June a pilot reported an ash cloud 8 km (26,000 ft) a.s.l. This ash cloud was not observed in satellite imagery due to dense weather clouds. On 24 June a VT earthquake of magnitude 2.3, was located 2.5 km S of the crater and a depth of 6.4 km (21,000 ft). On 23 June CENAPRED received reports of ash fall in Tetela del volcán and Ocuituco, municipalities of Morelos.

Based on information from CENAPRED, the Washington VAAC reported that on 19 April light gas emission and incandescence were visible in Popocatépetl's crater. Meteorological clouds obscured satellite views of the crater. The Washington VAAC reported that aircraft should avoid flying over Popocatépetl in case increased activity produced an ash plume.

During 9-15 March, Popocatépetl occasionally emitted steam, gas, and small amounts of ash. An ash emission on 9 March at 1304 produced a plume that rose to ~500 m above the summit crater and drifted E. Ash fell in the towns of Huejotzingo (~30 km NE of the volcano) and Puebla (~45 km E). The Alert Level at Popocatépetl remained at Yellow Phase I.

According to the Washington VAAC, an explosion at Popocatépetl on 22 January produced a NE-drifting ash plume. CENAPRED reported that aerial photographs taken on 14 January showed subsidence in the inner crater of Popocatépetl and no external lava dome at the bottom of the crater. Popocatépetl remained at Alert Level Yellow- Phase 1, with access restricted within a 12-km radius around the crater.

On 9 January, the Washington VAAC reported that an ash plume rose up to ~10 km a.s.l. from Popocatépetl and extended S. Satellite imagery indicated that by 10 January, ash clouds had reached the coast of Mexico to the volcano's S and SW.

On 26 May at 0643 a small emission from Popocatépetl of steam, gas, and small amounts of ash produced a plume that reached a height of ~1.5 km above the volcano's crater. Ash fell in Tetela del Volcán, Morelos. Also, an M 2.4 volcano-tectonic microearthquake occurred 2 km E of the crater. Aerial photography taken on 14 April showed continued subsidence of the inner crater. No external lava dome at the bottom of the crater was distinguished. The Alert Level at Popocatépetl remained at Yellow Phase 1, therefore access was restricted in a 12-km-radius around the volcano.

During December relatively low-level volcanic activity continued at Popocatépetl, with low-intensity steam-and-gas emissions occurring. An aerial photograph taken on 10 December showed subsidence of the inner crater and no external lava dome at the bottom of the crater. The Alert Level remained at Yellow Phase II.

Activity at Popocatépetl remained stable during the week with numerous gas-and-steam emissions. Based on information from the Mexico City MWO, the Washington VAAC reported that an ash plume was emitted on 22 November that rose to ~9.5 km a.s.l. and extended ~19 km NE.

This tall volcano is commonly obscured by clouds and scientists are then forced to rely on seismicity detected by the monitoring network to define the number of "daily exhalations." Daily exhalations during the week of 5-11 November 2003 typically stood at ~5. By comparison, during March 1996 the daily exhalations reached ~180. Other monitored parameters during 5-11 November also indicated relative stability. Scientists also consider the energy released by the exhalation-related seismicity, and not surprisingly, this also stood much higher during mid-1996. A 17 October 2003 flight over the volcano allowed scientists to see into the crater; the crater floor looked comparatively flat, without signs of a rising dome. Good visibility on 6 November brought little sign of fumarolic steam rising from the volcano. No aviation ash advisories were issued for Popocatépetl during this week.

During October, moderate emissions of mainly gas and steam occurred at Popocatépetl. Aerial photographs taken on 17 October showed that the inner crater had subsided and that no external lava dome was visible at the bottom of the crater.

During 17-23 September, moderate emissions of mainly gas, steam, and sometimes ash occurred at Popocatépetl. According to the Washington VAAC, on 21 September a small possible ash cloud was visible on satellite imagery at a height of ~7 km a.s.l., extending NW from the summit. The cloud dissipated rapidly.

Aerial photography taken on 25 August revealed no external lava dome in Popocatépetl crater. On 28 August the number of low-level emissions increased in comparison to previous days. On 29 August at 1330 a low-density ash emission rose to ~1.5 km above the crater and drifted W. There were no reports of ash fall in villages near the volcano. This event was accompanied by episodes of high-frequency and low-amplitude tremor. By 1 September the number of emissions returned to levels recorded prior to the increase.

During 23-29 July, moderate emissions of mainly gas, steam, and sometimes ash occurred at Popocatépetl. Aerial photography taken on 21 July revealed than an external lava dome was not visible at the bottom of the crater. A significant explosion occurred on 25 July at 2053, throwing incandescent fragments on Popocatépetl's slopes. According to news reports, the loud explosion panicked some residents in nearby communities.

During 16-22 July, moderate emissions of mainly gas and steam occurred at Popocatépetl. On 19 July at 0920 a moderate explosion produced an ash plume that rose ~3 km above the crater. After the event the volcano returned to its previous level of activity characterized by frequent and small steam-and-ash emissions. According to news reports, a small amount of ash fell in Mexico City. The international airport remained open, with minor disruptions to air traffic.

During 2-8 July, moderate emissions of mainly gas, steam, and sometimes ash, occurred at Popocatépetl. A relatively large ash emission occurred on 2 July at 0755 that produced an ash cloud to ~3.5 km above the crater. After the event, volcanism returned to previous levels characterized by frequent and small emissions of steam and gas.

During 25 June to 1 July, moderate emissions of mainly gas, steam, and sometimes ash, occurred at Popocatépetl. On 28 June the number of emissions was higher than on other days, and the plumes had a higher ash content. Ash fell in towns SW of the volcano.

During 18-24 June, moderate emissions of mainly gas and steam, and sometimes ash, occurred at Popocatépetl. In addition, isolated episodes of low-amplitude harmonic tremor were occasionally recorded. One of the more significant emissions, on 20 June at 0636, produced an ash-and-steam plume to a height of ~2 km above the volcano. Ash plumes were occasionally visible on satellite imagery.

CENAPRED reported that a small eruption at Popocatépetl on 10 June at 1744 produced a W-drifting ash column to a height of ~3 km above the volcano. In addition, episodes of low-amplitude harmonic tremor were recorded for a total of 2 hours. According to the Washington VAAC, the volcano returned to just gas venting after the eruption. Based on information from the México City MWO, the Washington VAAC also reported that a small emission occurred on 15 June at 1401. Aviators reported that the cloud from this eruption rose to ~2 km above the volcano and drifted N.

During 7-10 May, moderate emissions of mainly gas and steam occurred at Popocatépetl. In addition, isolated episodes of low-amplitude harmonic tremor were occasionally recorded. No signs of a new lava dome were seen during a flight over the volcano on 20 April.

On 24 April at 2050 Popocatépetl issued a moderate explosion that threw incandescent fragments 1.5-2.0 km above the crater and generated an ash column that rose to ~2.5 km above the crater. The plume was initially carried E. This activity was related to the destruction of a small dome that formed in the past few days. After the explosion, activity dropped to its previous more tranquil levels.

A small explosion occurred late on 26 April, followed by a clear increase in vigor on 28 April, including another moderate explosion at 0120 that day. Incandescent fragments landed on the E and NE flanks up to ~800 m from the crater. The outburst included a 40-second phase of greatest intensity. Ash falls were not reported. Hours later on 28 April instruments began to register harmonic tremor of moderate amplitude, signals that continued for 13 hours.

During the next day the eruptive vigor dropped to modest levels (only four exhalations of low intensity occurred, a smaller number than typical, and chiefly containing steam and gas rather than ash). Scientists attributed the 28 April eruption and increased eruptive vigor to advanced rates of lava ascent and extrusion. They suggested that these may have been higher than seen either earlier this year or near the end of last year. The hazard status was raised one increment, to Yellow phase 3. In addition, the highway traversing the Cortés pass was closed to traffic for two days, 29-30 April.

The ongoing eruption at Popocatépetl was punctuated by a small explosion on 17 April, an event accompanied by incandescent fragments that reached 1 km E along the summit crater=s outer margin, and a modest ash plume directed toward the NE. Associated with the event, low-amplitude tremor persisted for about 2 hours, but no other significant geophysical changes were seen. More typical low-intensity outbursts also continued during the week, typically at rates of ten=s per day. The alert level still stood at Yellow, and the mountain road across the N flank remained open.

During 9-15 April, moderate emissions of mainly gas and steam occurred at Popocatépetl. In addition, isolated episodes of low-amplitude harmonic tremor were occasionally recorded. There were still remains of the lava dome in Popocatépetl's crater.

During 2-8 April, moderate emissions of mainly gas and steam occurred at Popocatépetl. In addition, isolated episodes of low-amplitude harmonic tremor were occasionally recorded. There were still remains of the lava dome in Popocatépetl's crater.

Several emissions of steam, gas, and ash occurred during 12-18 March associated with the continued destruction of Popocatépetl's lava dome. The largest reported emission, on 13 March at 0916, produced a steam-and-ash cloud that reached a height of 1 km above the crater and drifted E. According to a news article, ash fell on several communities near the volcano.

Several emissions of steam, gas, and ash occurred during 5-11 March associated with the continued destruction of Popocatépetl's lava dome. The largest reported emission, on 5 March at 1005, produced a steam-and-ash cloud that reached a height of 1 km above the crater. Also, a microearthquake and isolated episodes of low-amplitude harmonic tremor were recorded. The Alert Level at Popocatépetl remained at Yellow Phase II, with a 12-km-radius restricted area.

Several emissions of steam, gas, and ash occurred during 26 February to 4 March associated with the continued destruction of Popocatépetl's lava dome, although there were fewer larger emissions than the previous week. The largest reported emission occurred on 28 February at 0314, with incandescent volcanic fragments ejected ~1 km around the crater and an ash emission that initially drifted E.

Several emissions of steam, gas, and ash occurred during 19-25 February associated with the continued destruction of Popocatépetl's lava dome. One of the larger emissions occurred on 21 February at 1820 and produced a NE-drifting ash cloud to a height of 4 km above the volcano. It was followed by an emission the next day at 0239, which produced a NE-drifting ash cloud to 2 km above the volcano. Both emissions ejected incandescent fragments ~1.5 km from the volcano, causing fires in pasture land. According to the Washington VAAC, emissions on the 22nd produced two ash plumes that were visible on satellite imagery; one extended from the central Gulf of Mexico to the Yucatan Peninsula at a height ~10.7 km a.s.l., and a second smaller plume was over the Bay of Campeche at ~9.4 km a.s.l. Later that day, ash was visible on satellite imagery extending from Lake Okeechobee, Florida, NE into the Atlantic Ocean.

A moderate explosion at Popocatépetl on 14 February at 0534 ejected incandescent fragments as far away as 3 km around the crater. Also, a dense ash column rose 5 km above the volcano and drifted NE. The ejected fragments caused some fires in pasture land. This eruption was related to the partial destruction of the lava dome. After the eruption, activity returned to low-to-moderate emissions of steam, gas, and ash.

During 4-10 February, several moderate-sized emissions at Popocatépetl sent ash plumes to a height of ~2 km. On 4 February at 0459 a moderate dome-destruction explosion ejected incandescent volcanic material that fell as far as ~2 km down the volcano's flanks. On 5 and 6 February similar sized emissions occurred that were accompanied by episodes of low-amplitude harmonic tremor for up to 3 hours. According to CENAPRED, due to the remains of a lava dome inside the crater, there remained a significant chance of further explosive activity, ash emissions, and incandescent ejections around the crater.

Volcanic activity continued at low-to-moderate levels at Popocatépetl during 28 January to 3 February. Activity consisted of small-to-moderate emissions of steam, gas, and small amounts of ash, and sporadic episodes of low-amplitude harmonic tremor. A moderate-sized emission occurred on 2 February at 0826 that produced an ash column to a height of ~2 km above the volcano. In addition, there were episodes of harmonic low-amplitude tremor during 2 hours.

Volcanic activity continued at low-to-moderate levels at Popocatépetl during 21-27 January. Activity consisted of small-to-moderate emissions of steam, gas, and small amounts of ash, and sporadic episodes of low-amplitude harmonic tremor. On 22 January at 0735 a significant increase in volcanic microsesimicity was recorded. According to the Washington VAAC, on the 25th an ash emission reached a height of ~10.7 km a.s.l. CENAPRED reported that the recorded seismic and volcanic activity were probably associated with the growth of a new lava dome inside the crater.

Volcanic activity continued at low-to-moderate levels at Popocatépetl during 15-20 January. Activity consisted of small-to-moderate emissions of steam, gas, and small amounts of ash, and sporadic episodes of low-amplitude harmonic tremor. On 9 January photographs of the lava dome revealed that the dome's inner crater had subsided.

The lava dome's volume was calculated to be approximately 500,000 m3. Explosive activity after the 15th was probably associated with the growth of a new lava dome. CENAPRED stated that in the next days moderate explosive activity, with ash and incandescent material emission, could occur.

Based on information from the México City MWO, the Washington VAAC reported that a thin plume was visible on satellite imagery on 1 January at 1245. According to the MWO, the plume was at a height of ~7.3 km a.s.l.

A moderate explosion occurred at Popocatépetl on 23 December. The resultant ash cloud rose ~2 km above the volcano and deposited ash on nearby towns. CENAPRED indicated that this type of activity is related to the destruction of the lava dome. According to a news article, a series of small eruptions just before Christmas deposited ash in towns as far away as central Texas. According to the article, "Upper-level winds just happened to be flowing from the Pacific Ocean over central México towards Texas, and that 'jet stream' effectively transported the ash right into central Texas." There were reports of ashfall from San Antonio to New Braunfels, in addition to Austin, covering cars and vegetation with a light, white film.

Moderate explosions occurred at Popocatépetl on 18 December. After a period of high-frequency tremor, an explosion occurred at 0148 that produced an ash cloud that reached 2 km above the crater. Later, at 0209 and 0214 two explosions sent ash to ~0.5 km above the crater. that drifted to the NE. Fragments were sent around the crater a distance of ~1.5 km. The volcano returned to its previous relatively low level of activity. Popocatépetl remained at Alert Level Yellow phase II.

During December there were several emissions of steam, gas, and minor amounts of ash at Popocatépetl. According to CENAPRED, new episodes of low-frequency tremor, beginning on 19 November, signaled the growth of a new lava dome within Popocatépetl's crater. Aerial photographs obtained by the Mexican Ministry of Communications and Transportation on 2 December confirmed the presence of a fresh lava dome measuring about 180 m in diameter at its base, and about 52 m high. The lava dome grew in episodes of variable duration, at a mean extrusion rate of 8-9 m3 per second.

These dome-growth episodes were distinctly recorded by the monitoring network as harmonic tremor. Small associated inflation was also recorded. CENAPRED stated that dome growth may continue and could conclude with dome-destruction episodes, which have occurred in the past. The Alert Level at Popocatépetl remained at Yellow.

A moderate eruption occurred at Popocatépetl on 6 November at 0735. According to aircraft reports, the eruption produced a small amount of ash that reached 4 km above the crater and drifted slightly to the N. An intense 3-minute-long phase was followed by high-frequency tremor. Minor ashfall occurred in towns including San Juan Tehuixtitlán, San Pedro Nexapa, Amecameca, Ecatzingo,Tepejomulco, Ozumba, and San Vicente Chimalhuacán. Four small eruptions also occurred during the day. The Alert Level remained at Yellow Phase II, with a restricted area of 12 km from the crater.

During September until at least 22 October, there were several emissions of steam, gas, and minor amounts of ash at Popocatépetl. Based on aerial photos taken on 17 September, the lava dome within the inner crater appeared to have subsided.

In August there were several emissions of steam, gas, and minor amounts of ash at Popocatépetl. In addition, episodes of both harmonic and high-frequency tremor occurred. CENAPRED attributes this activity to a new lava dome that has been growing for several weeks.

During 23-30 July, there were small-to-moderate emissions of steam, gas, and ash at Popocatépetl. Based on information from the Mexico City MWO, the Washington VAAC reported that on 26 July an ash cloud rose 5.5-6.7 km a.s.l.

During 26 June-1 July, there were small-to-moderate emissions of steam, gas, and ash at Popocatépetl. In addition, several episodes of low-amplitude harmonic tremor were recorded. The most notable emissions, on 26 June at 1110 and on 1 July at 0238, produced ash clouds to a height of ~2 km above the volcano.

During 12-18 June, activity at Popocatépetl mainly consisted of small steam-and-gas emissions. A photograph of the lava dome taken on 22 May revealed that it had diminished in size compared to 29 April. Based on information from ground reports, video footage, and MWO, the Washington VAAC reported that an eruption occurred on 17 June at 1140. An ash cloud was produced that rose to a height of ~8 km a.s.l. and drifted to the WSW.

During 21-28 May, Popocatépetl emitted small clouds of steam, gas, and generally minor amounts of ash. In addition, episodes of harmonic tremor were recorded. On 21 May at 1330 a small explosion occurred that was followed by 15 minutes of high-frequency tremor. According to a news report, a small amount of ash fell on communities neighboring the volcano on 22 May.

During 8-12 May, Popocatépetl emitted small clouds of steam, gas, and generally minor amounts of ash. In addition, episodes of harmonic tremor were recorded. A small explosion on 12 May at 0609 hurled incandescent pyroclasts less than 500 m down the volcano's N flank. Aerial photography taken on 29 April by the Department of Federal Roads and the Ministry of Communication and Transportation revealed a 170-m-diameter lava dome growing in the crater. According to CENAPRED, recent volcanic and seismic activity at Popocatépetl has been related to the growth of this lava dome.

During 24-30 April, Popocatépetl emitted small clouds of steam, gas, and generally minor amounts of ash. In addition, episodes of harmonic tremor were recorded. On 29 April at 2232 an eruption produced an ash cloud to a height of 1.5 km and sent incandescent fragments 500 m from the crater. No ash was visible on satellite imagery.

During 3-9 April, Popocatépetl emitted small clouds of steam, gas, and generally minor amounts of ash. In addition, episodes of harmonic tremor were recorded. A moderate explosion on 8 April at 0438 produced an ash cloud to a height of ~7.6 km a.s.l. Incandescence was visible at the volcano during the eruption. A small ash cloud was visible on satellite imagery drifting to the E and by 1145 the cloud had drifted over the coast of México near the city of Veracruz.

During 27 March-2 April, Popocatépetl emitted small clouds of steam, gas, and generally minor amounts of ash. In addition, episodes of harmonic tremor were recorded. Based on information from the Mexico City MWO, the Washington VAAC reported that on 26 March at 2132 seismicity suggested that an emission may have occurred. Extensive cloud cover precluded the detection of ash on satellite imagery.

During 6-11 March, Popocatépetl emitted small clouds of steam, gas, and generally minor amounts of ash. In addition, episodes of harmonic tremor were recorded. On 9 March a steam-and-gas emission rose to ~2 km above the crater.

During 28 February-5 March, Popocatépetl emitted small clouds of steam, gas, and generally minor amounts of ash. In addition, episodes of harmonic tremor were recorded. According to CENAPRED, volcanic and seismic activity during the report period was probably related to a new episode of lava-dome growth.

On 19 February frequent small-to-moderate emissions of steam, gas, and minor amounts of ash occurred at Popocatépetl. In addition, small explosions occasionally threw incandescent volcanic fragments short distances from the crater. During 0750-0900 small amounts of ash fell in Paso de Cortés. Also, isolated episodes of harmonic tremor were registered. On 23 February at 2156 a small explosion threw incandescent fragments up to 200 m that fell within the crater and a low-volume ash column rose 700 m above the crater and drifted to the W. Later, isolated harmonic tremor was detected. According to CENAPRED, the activity on the 19th and 23rd was related to the partial destruction of the lava dome in the crater and low-level explosive activity was considered possible in the next days or weeks.

Seismicity increased slightly at Popocatépetl during 15-16 February after several weeks of low activity. Harmonic tremor and low-magnitude volcanotectonic micro-earthquakes were recorded. During the same period emissions of gas, steam, and some ash occurred. CENAPRED stated that the activity was possibly related to the ascent of magma and the formation of a new lava dome. They added that this activity could lead to explosions in the next days to weeks. The volcano remained at Alert Level Yellow Phase II.

Air photos taken on 24 January revealed that the new lava dome growing in Popocatépetl's summit crater was 180 m in diameter and 150 m high. During 30 January-5 February Popocatépetl emitted small clouds of steam, gas, and generally minor amounts of ash. In addition, episodes of harmonic tremor were recorded.

According to CENAPRED, on 23 January at 0517 a moderate eruption began at Popocatépetl. It was accompanied by continuous tremor. Incandescent volcanic fragments were ejected short distances from the crater, and a steam-and-ash cloud was produced. According to pilot reports, the cloud rose to ~9 km a.s.l. Satellite imagery showed that the cloud drifted to the NE; by 1845 it was visible near the city of Poza Rica, ~250 km to the NE. The cloud continued drifting NE and traveled over the Gulf of Mexico. Small amounts of ash fell in Paso de Cortés and the town of Tlaxcala. CENAPRED reported that the activity was related to the formation of a new lava dome. Following the 23 January eruption, Popocatépetl emitted small clouds of steam, gas, and generally minor amounts of ash. In addition, episodes of harmonic tremor were recorded.

During the week, Popocatépetl emitted small clouds of steam, gas, and generally minor amounts of ash. In addition, episodes of harmonic tremor were recorded. The highest reported volcanic cloud, from an eruption on 13 January at 1709, reached a height of ~7.3 km and drifted to the ENE. A photograph of Popocatépetl 's summit area taken on 11 January confirmed that the lava dome reported in December 2001 had been partially destroyed.

During the week, Popocatépetl emitted small clouds of steam, gas, and generally minor amounts of ash. In addition, episodes of harmonic tremor were recorded. On 7 January at 1330 a small ash emission was visible on satellite imagery at a height of 7.6 km a.s.l.

During the week, Popocatépetl emitted small clouds of steam, gas, and generally minor amounts of ash. In addition, episodes of harmonic tremor were recorded. The highest reported ash cloud was produced from an eruption on 29 December at 0432 and rose 5.5-8.8 km a.s.l.

A period of lava-dome destruction and possible new dome growth during 18 to at least 26 December included several explosions and episodes of harmonic tremor. During a 3-minute-long eruption on 19 December starting at 1926, volcanic fragments were hurled 2 km to the E and NE. In addition, an ash cloud was probably produced, but was obscured by meteorological clouds. Another notable explosion, on 22 December at 1735, produced an ash cloud that rose 2.5 km above the volcano and drifted to the NE. After this event a small amount of ash fell in the town of Puebla, ~50 km E of the volcano. By 23 December volcanic activity decreased, with fewer eruptions, less fumarolic activity, and short episodes of high-frequency tremor. According to CENAPRED, the activity during the week was related to destruction of the lava dome first seen on 21 November 2001. They warned that similar activity may occur in the following days or weeks. The volcano remained at Alert Level Yellow Phase II.

During the week, Popocatépetl emitted small clouds of steam, gas, and generally minor amounts of ash. In addition, episodes of harmonic tremor were recorded. The highest reported ash cloud was produced from an eruption on 17 December at 2213. It rose 7.3 km a.s.l. and drifted to the SW. CENAPRED reported that a new 190-m-diameter lava dome in the volcano's crater was observed on 11 December.

On 10 December at 1830 an increase in activity began at Popocatépetl. There were more ash emissions at the volcano than in comparison to the previous weeks. Several small-to-moderate emissions ejected incandescent fragments about 1 km around the volcano. The strongest activity occurred during 2000 to 2200. Following the increased activity only sporadic emissions occurred. The Washington VAAC reported that ash clouds rose less than 1 km above the volcano and drifted to the ENE.

During the week, Popocatépetl emitted small clouds of steam, gas, and minor amounts of ash. In addition, episodes of harmonic tremor were recorded. During a flight over the volcano on 21 November observers noticed a new 130-m-diameter lava dome growing inside the internal crater. The growth of this lava dome corresponded with increased emissions and seismicity on 17 and 18 November.

During the week, Popocatépetl emitted small clouds of steam, gas, and minor amounts of ash. There were episodes of increased emissions and seismometers recorded periods of harmonic tremor. According to CENAPRED, this type of activity may be associated with lava dome growth, as occurred on 15 August and 10 September 2001.

On 10 November at 0324 a small explosive event produced an ash column that rose 1 km above Popocatépetl's summit and drifted to the E. CENAPRED's volcano camera showed incandescent fragments hurled up to 300 m E. During the rest of the week, volcanic activity at Popocatépetl remained low, with emissions of small clouds of steam, gas, and minor amounts of ash.

Volcanic activity remained low at Popocatépetl, with emissions of small clouds of steam, gas, and minor amounts of ash. Episodes of low-amplitude harmonic tremor also occurred. An aerial photograph of the crater was taken on 25 October by the Dirección General de Carreteras Federales, SCT. It showed that the lava dome, which was first reported on 20 September 2001, had subsided. In addition, a new, small 50-m-diameter lava dome had grown in the bottom of the internal crater. According to CENAPRED, a similar situation had been observed in April 2001 and the presence of the lava dome indicates the possibility of small explosions occurring in the next days to weeks. The volcano remained at Alert Level Yellow Phase II.

On 25 October at approximately 0600 a steam column was observed rising 1-2.5 km above Popocatépetl's crater, drifting to the NW. Ground observations confirmed that the column did not include ash. The MWO reported to the Washington VAAC that another eruption column the same day at 1040 rose to ~6.7 km. A narrow plume of ash from the eruption was visible extending to the N on satellite imagery. The volcano remained at Alert Level Yellow Phase II.

During the week, Popocatépetl emitted small clouds of steam, gas, and minor amounts of ash, and underwent episodes of harmonic tremor. According to reports from the México City MWO to the Washington VAAC, a small eruption on 9 October at 0712 produced an ash cloud that rose 2 km above the volcano and drifted to the W.

During the week, volcanic activity at Popocatépetl consisted of small emissions of steam, gas, and minor amounts of ash, and episodes of harmonic tremor. Pilots reported that a steam emission rose 4 km above the volcano on 26 September at 1000.

During the week, volcanic activity at Popocatépetl consisted of small emissions of steam, gas, and minor amounts of ash, and episodes of harmonic tremor. Analysis of aerial photographs taken on 20 September revealed that the lava dome had continued to grow since it was first observed on 10 August. The dome is growing inside the inner crater, which formed within the summit crater after a dome was destroyed in December 2000.

On 9 September at about 0815 an episode of frequent moderate-sized eruptions began at Popocatépetl. The eruptions produced steam-and-ash emissions that rose to a maximum height of 1 km above the crater and drifted to the NW. During the night a small explosion sent incandescent fragments up to 200 m from the crater. Eruptive activity continued through at least 1230 on 9 September.

Small emissions of steam, gas, and small amounts of ash continued at Popocatépetl. The Mexico City MWO reported that on 17 August at 1514 an ash emission produced a cloud that rose to 7.3 km a.s.l. The Washington VAAC reported that GOES-8 satellite imagery did not show an ash plume, but did show an occasional hotspot. CENAPRED reported that recent activity was related to the growth of a new lava dome inside the crater. The volcano remained at Alert Level Yellow Phase II.

The number of small emissions composed of steam, gas, and small amounts of ash increased in comparison to the previous week. The Washington VAAC reported an ash emission on 9 August at 2300 produced an ash cloud that rose to 7.6 km a.s.l. According to CENAPRED, the increase in activity may have been related to several days of intense rain in the area.

Several small-to-moderate sized emissions occurred at Popocatépetl that were mainly composed of steam, gas, and small amounts of ash. The Washington VAAC received a pilot report on 24 July at 1100 of an ash cloud ~5.5 km above the volcano drifting to the W. On 23 July CENAPRED reduced the Alert Level from Yellow Phase III to Phase II because volcanic activity was at lower levels than it was in December 2000 when the Alert Level was originally raised.

Several small-to-moderate sized emissions occurred at Popocatépetl, including three on 14 July. According to the Washington VAAC, around 1045 an emission of steam, gas, and small amounts of ash rose to ~7.3 km a.s.l. and drifted to the W. Emissions also occurred at 2303 and 2341. A cloud from the latter emission rose to ~6.1 km a.s.l. and drifted to the W.

On 3 July at 0410 and 0648 moderate-sized explosions occurred. The latter explosion lasted about 10 minutes and produced an ash cloud that rose ~4 km above the volcano. Initially the cloud drifted to the SE and later the highest portion of the cloud drifted to the NW. Based on information from pilot reports and ground observations, the Washington VAAC reported that the ash cloud was 9.3 km SE of Mexico City Airport at 0930. Ashfall occurred in several towns including Chalco, ~35 km NW of the volcano, and there were reports of light ashfall on the airport's runways.

Volcanic activity at Popocatépetl remained at normal levels, with several small exhalations of steam, gas, and small amounts of ash. Based on information from the Mexico City MWO, the Washington VAAC stated that on 1 July at 0915 a small eruption produced an ash plume that rose to less than 1 km above the volcano and drifted to the SSW. On 3 July at 0425 a moderate-sized exhalation produced an ash cloud seen on satellite imagery to spread in two directions; to less than 1 km above the volcano drifting to the NW, and ~4 km above the volcano drifting to the SE.

Volcanic activity at Popocatépetl remained at normal levels, with several small exhalations of steam, gas, and small amounts of ash. Based on information from CENAPRED, the Washington VAAC reported that on 24 June at 0900 a small eruption produced an ash cloud that rose ~0.5 km above the volcano and drifted to the SW.

Volcanic activity at Popocatépetl remained at normal levels, with several small exhalations of steam, gas, and small amounts of ash. The volcano remained at Alert Level Yellow Phase III, with a restricted 12-km-radius area.

Activity during 6-7 June remained at normal low levels, with minor gas-and-steam emissions, tremor, and a few small volcano-tectonic earthquakes at around 6 km depth below the crater. Increased emissions on 8 June sometimes included small amounts of ash. An explosion on 9 June at 0424 sent ash to an unknown height. Moderate activity continued through the next morning and decreased slightly. The Mexico City MWO reported an ash emission on 11 June at 1100 that rose 7.6 km a.s.l., but it was not seen on satellite imagery. The MWO warned of another ash emission on 12 June at 1648, but cloudy conditions prevented a height estimate. Typical low-to-moderate activity continued through 13 June, with the Yellow alert level unchanged.

CENAPRED reported that on 31 May at 2136 a moderate-sized eruption began with the most intense phase lasting ~1 minute. Incandescent material traveled 2-3 km down the NE flank of the volcano. According to the Mexico City MWO a steam-and-ash cloud was observed rising up to 7.6 km a.s.l. and drifting to the W. A smaller eruption occurred on 1 June at 0804 that sent a steam-and-ash cloud up to 7 km a.s.l.

Small-to-moderate sized exhalations occurred during the week. Based on information from the Mexico City MWO, the Washington VAAC reported that an eruption occurred on 26 May at 1122 that sent a steam-and-ash plume up to ~7 km a.s.l.

Several small exhalations at Popocatépetl produced small plumes of steam and ash with the largest plume rising up to 1.5 km above the crater. The volcano remained at Alert Level Yellow Phase III, with a restricted 12-km-radius area.

CENAPRED reported that at 0819 on 29 April a moderate explosion occurred at Popocatépetl that lasted for 1 minute and produced an ash cloud that rose 2 km above the volcano's summit and drifted to the ENE. A pilot reported that the ash cloud reached an altitude of 9 km a.s.l. Light ash fall was reported in San Pedro Benito Juárez ~10 km SE of the volcano's summit. Throughout the day several episodes of harmonic tremor occurred. Based on information from the Mexico City MWO, the Washington VAAC reported that another eruption occurred later the same day at 1310. It produced an ash cloud that rose 6.7-7.6 km a.s.l. The volcano remained at Alert Level Yellow Phase III, with a restricted 12-km-radius area.

Small-to-moderate exhalations continued at Popocatépetl. On 17 April a small lahar traveled down the Achupashal Gorge. The volcano remained at Alert Level Yellow Phase III, with a restricted 12-km-radius area.

Several small-to-moderate exhalations occurred during the week at Popocatéptl. CENAPRED reported that at 1948 on 16 April a moderate explosion sent incandescent fragments up to 2 km away from the volcano's crater to the NE and NW and produced an ash plume that rose 4 km above the crater and drifted to the SW. The 40-second-long eruption partially destroyed the lava dome that had formed within the crater over the course of several weeks. The volcano remained at Alert Level Yellow Phase III, with a restricted 12-km-radius area.

The CENAPRED reported that during the previous two weeks activity increased at Popocatépetl. Seismographs detected episodes of harmonic tremor totaling up to 8 hours per day, two tectono-volcanic earthquakes per day with magnitudes up to 2.3, and high-frequency tremor. This activity is related to the emplacement of a new lava dome, which was first observed on 14 March, that could produce explosive activity. The volcano remained at Alert Level Yellow Phase III, with a restricted 12-km-radius area.

CENAPRED reported that volcanic activity continued at Popocatépetl, with several small exhalations and moderate seismic activity. The volcano remained at Alert Level Yellow Phase III, with a restricted 12-km-radius area.

According to CENAPRED, volcanic activity remained low and relatively unchanged throughout 22-27 March. Low-intensity steam-and-gas exhalations continued, sometimes with small amounts of ash. Spoardic episodes of harmonic tremor were reported on most days, with the longest episode lasting 1.5 hours on the evening of 24 March. The volcano remained at Alert Level Yellow Phase III, with a restricted 12-km-radius area.

According to CENAPRED, volcanic activity that was relatively high during 14-16 March began to diminish on 17 March. On 14 and 15 March there were many small exhalations of steam, ash, and gas, as well as episodes of harmonic tremor that totaled 1 hour. On 15 March a new lava dome ~200 m in diameter and 40 m high was observed at the volcano's summit. By 17 March fewer exhalations occurred than on previous days and harmonic tremor was only detected for a total of 15 minutes. The volcano remained at Alert Level Yellow Phase III, with a restricted 12-km-radius area.

CENAPRED reported that Popocatépetl's activity was at low-to-moderate levels during most of the week, with small exhalations accompanied by steam emissions. Based on reports from the Mexico MWO, the Washington VAAC reported that at 2024 on 12 March an ash cloud from an exhalation of Popocatépetl was observed at a height of ~ 7 km a.s.l. The ash cloud was not visible on GOES-8 imagery. The volcano remained at Alert Level Yellow Phase III, with a restricted area of 12-km-radius.

CENAPRED reported that activity remained relatively low at Popocatépetl during the week, with small exhalations accompanied by steam emissions. The volcano remained at Alert Level Yellow Phase III, with a restricted area of 12-km-radius.

CENAPRED reported that small exhalations occurred at Popocatépetl during the week. The Washington VAAC did not report that ash was visible in satellite imagery. The volcano remained at Alert Level Yellow Phase III, with a restricted area of 12-km-radius.

The Washington VAAC reported that an eruption occurred at 1642 on 15 February, producing an ash cloud that rose up to 7.9 km a.s.l. and drifted to the ENE. The cloud was visible on GOES-8 imagery and by 1802 it had traveled to the Gulf of Mexico. The volcano remained at Alert Level Yellow Phase III, with a restricted area of 12-km-radius.

The Washington VAAC reported several small ash-bearing eruptions during the week. At 1921 on 8 February an eruption produced a small ash cloud that rose up to ~7.6 km a.s.l. and blew to the NE. At 1400 on 9 February an eruption produced an ash cloud that rose up to ~6.7 km. A small ash plume produced from emissions that occurred at 1338 and 1348 on 11 February was visible in GOES-8 imagery. The ash plume rose up to ~7.9 km a.s.l. and blew to the S.

The Washington VAAC reported that during 1530 to 1545 on 30 January a moderate ash emission was visible on the CENAPRED camera; it rose to ~7 km a.s.l. and blew to the NNE. At 1345 on 1 February a small eruption was visible on GOES-8 imagery. The narrow plume of ash rose to ~6.7 km and blew to the NNE. The volcano remained at Alert Level Yellow Phase III, with a restricted area of 12-km-radius.

During the week several small-to-moderate sized eruptions occurred, with light ashfall reported in two towns and a pyroclastic flow stopping 8 km short of a town. The Washington VAAC reported that on 25 January an ash plume, which was produced by rockfall activity, rose to ~7 km a.s.l and was visible on GOES-8 imagery. According to CENAPRED, at 1338 on the same day an exhalation produced an ash cloud that rose to 3 km above the volcano and blew to the NW, depositing ash in San Pedro Nexapa, ~15 away. A minor eruption at 1212 on 27 January produced an ash cloud that rose to 6.4 km a.s.l. and blew to the NE, depositing light ash in the town of Santiago Xalitzintla, ~15 km from the volcano. An ash-and-steam eruption at 1155 on 28 January produced an ash cloud that that rose to 7 km and blew to the NE. Based on information from the Mexico City MWO, the Washington VAAC reported that an eruption occurred at 1104 on 29 January. A pilot report stated that the ash cloud from the eruption rose to ~11.9 km a.s.l., while CENAPRED reported that the cloud rose to ~8 km a.s.l. and blew to the NE. The eruption sent pyroclasts out to 1 km from the crater and produced pyroclastic flows that traveled down the NE flank of the volcano, stopping 8 km before reaching the town of Santiago Xalitzintla. The pyroclastic flows caused some melting of the summit glacier located primarily on the upper N and W flanks. The volcano remained at Alert Level Yellow Phase III, with a 12-km-radius restricted area.

In addition to multiple exhalations, Popocatépetl produced both pyroclastic flows and mudflows that traveled several kilometers downslope. The Washington VAAC reported that at ~1500 on 14 January a small exhalation sent ash to ~7.6 km a.s.l. They also reported that a larger explosive eruption occurred at ~1615 on 22 January that sent ash to 6-12 km a.s.l. CENAPRED reported that the eruption produced pyroclastic flows that descended ~4-6 km down several gorges on the N and NW flanks of the volcano. Ash was deposited on Santiago Xalitzintla, Atlixco, Tecamachalco, Tetela, and part of Puebla. A small (10 cm thick and 2 m wide) mudflow traveled up to 8 km from the town of Santiago Xalitzintla down the Huiloac gorge. Scientists believe the pyroclastic flows melted a small portion of the glacier near the volcano's summit and the glacial meltwater mixed with ash. Another explosive eruption that occurred at 1915 the same day was followed by ongoing ash exhalations through 23 January. An eruption at 1041 on 23 January sent ash to ~9.1 km a.s.l. that blew to the S. Several small explosions and continuous ash emissions followed. The volcano remained at Alert Level Yellow Phase III with a 12 km security radius.

CENAPRED reported that low-level activity continued at Popocatépetl. The Washington VAAC remotely detected two exhalations on GOES-8 imagery during the week; at 1300 on 4 January a small eruption sent ash to ~7.3 km a.s.l. that blew to the E, and at 0655 on 8 January a brief ash-and-steam emission sent a cloud to ~6 km a.s.l. that blew to the E. The volcano remained at Alert Level Yellow Phase III, with a 12-km-radius restricted area.

CENAPRED reported that low-intensity exhalations occurred during most of the week. At 1118 on 30 December an intense seismic signal was recorded for 4 minutes that was characteristic of exhalations with explosive initial phases. At 1955 on 30 December a pilot report stated that an eruption sent an ash cloud to 8.5 km a.s.l. CENAPRED received reports that ash fell in the towns of Huejotzingo (~30 km to the NE of the volcano), San Pedro (~10 km to the SE), Cholula (~35 km to the E) and Puebla (~50 km to the E). GOES-8 imagery showed that ash from the eruption dissipated by 0100 on 31 December. Exhalations also occurred at 0507 on 31 December and at 0936 on 2 January, and sent ash clouds to 8.5 and 7.6 km, respectively.

Volcanic activity at Popocatépetl decreased in comparison to last week. The Washington VAAC reported that during 21 to 25 December there were intermittent emissions of mostly steam. In addition, several small-to-moderate explosive events occurred during the week; at 1555 on 24 December ash was erupted to ~9 km a.s.l. and then blown to the E; at 1045 on 25 December ash was erupted to between 5.5 and 7.6 km a.s.l., blown to the NE, and deposited less than 5 km from the summit; and moderate exhalations occurred at 0111 and 0631 on 27 December that sent ash up to 7.6 and 9.8 km, respectively. On 26 December CENAPRED reported that beginning on 24 December the volcano entered a new phase of activity. Moderate explosions were expected to continue for several days or weeks until the lava dome in the summit crater is destroyed. CENAPRED scientists determined that there was decreased likelihood of a large eruption. Most of the 41,000 residents near the volcano, who were evacuated beginning on 15 December, were permitted to return to their homes. The evacuees were warned to remain alert for further activity. The director of CENAPRED, Robert Quaas, told journalists at a press conference on 26 December that, "The volcano could continue to launch incandescent fragments as far as 5 km and could provoke a moderate rain of cooled fragments as far away as 10 km, because of events related to the destruction of the lava dome." The volcano remained at Alert Level Yellow Phase III, but the restricted area was reduced from 13 to 12 km. For more information about the present state of the volcano and the return of evacuees to their homes refer to CENAPRED's 26 December Bulletin.

For the first time in 6 years, continuous-fountaining Strombolian-style eruptions of incandescent spatter occurred at Popocatépetl, marking a significant change in eruption behavior. In addition, strong harmonic tremor and high-mass flux rates have occurred. As of the end of the report period, the summit crater contained hot lava, and continuous ash emissions deposited ash in several towns surrounding the volcano. This report is updated weekly but volcanic activity can change rapidly. To obtain more up-to-date volcanic activity reports refer to the CENAPRED website and/or contact their hotline at 01-800-123-5050. In addition, the Washington VAAC website provides detailed accounts of ash-producing volcanic activity.

Since the large exhalation on 12 December sent ash to an altitude of ~10.6 km a.s.l., continuous ash emissions have occurred, with occasional large exhalations of ash reaching a maximum altitude of ~11 km a.s.l. High levels of seismic activity, including harmonic tremor, and other monitoring parameters led officials to raise the Alert Level at the volcano on 16 December from Yellow Phase II to Phase III, thus increasing the high-risk zone from 7 to 10 km. In addition, according to a Reuters article, officials called on ~30,000 residents living within a ~12-km-radius around the volcano to evacuate to 180 refugee centers. On 16 December CENAPRED scientists flew over the volcano and found that a new lava dome had formed in the crater. The Washington VAAC reported that an eruption involving lava began at 1915 on 18 December, sending an ash cloud to an altitude of ~7.3 km a.s.l. In addition, the eruption sent incandescent spatter 1 to 2 km SE of the volcano. CENAPRED reported that during 18 and 19 December a large increase in volcanic activity occurred, consisting mainly of Strombolian ejections. According to a Reuters article, there were concerns that a ~900-km-long glacier on the volcano's western face could be melted by lava and produce lahars. This led officials to extend the high-risk zone to ~20 km. Interior Minister Santiago Creel told a news conference on 19 December, "We are on maximum alert ... because we have to be ready for any possible contingency regarding the glacier." As of 20 December (the end of the report period) continuous ash emissions were occurring and a very large ash cloud covered a large portion of S Mexico, extended E across the southern Gulf of Mexico and to near the N tip of the Yucantan Penninsula.

A reduction in volcanic activity at Popocatépetl led CENAPRED to reduce the Alert Level on 6 December from Yellow Phase III to Phase II, thus reducing the high-risk zone from 10 to 7 km. The Washington VAAC reported that a small steam-and-ash exhalation sent an ash cloud to ~6.7 km a.s.l. at ~2034 on 6 December. Later in the week they reported that at 1607 on 12 December a massive exhalation of ash and steam sent a cloud to ~ 8.8-10.6 km a.s.l. The ash cloud was visible on CENAPRED's Popocatépetl "Web Cam" and GOES-8 imagery. The eruption ended by approximately 1830 and by 1845 GOES-8 imagery showed that the ash cloud extended 37 km to the NE and was 14 km wide. Imagery through 2315 showed two main areas of ash; the most dense area was at an altitude of ~10.6 km a.s.l., and the other area was between 4.9 and 5.5 km a.s.l. The local airport was alerted to the eruption and as of 12 December the volcano remained at Alert Level Yellow Phase II.

Volcanic activity continued at a high rate at Popocatépetl, with several small-to-moderate exhalations and eruptions. Remote-sensing data, aviation sources, and CENAPRED provided more details. One of the larger series of eruptions occurred during 0900 to 1215 on 29 November, sending ash-and-steam plumes to ~7.3 km a.s.l to the ENE. Another moderate eruption at 1055 on 30 November sent an ash cloud to ~7.3 km a.s.l. GOES-8 imagery showed that by 1815 the cloud extended at least 204 km to the ENE and traveled over the Bay of Campeche, which is ~300 km to the E of the volcano. Between 0345 and 0402 on 4 December an eruption occurred that sent ash to ~7.6 km a.s.l. Throughout the week frequent exhalations sent ash to ~6-7.6 km.

Volcanic activity continued at a high rate at Popocatepetl, with several small-to-moderate exhalations and eruptions. Remote-sensing and aviation sources provided more details. The Mexico City MWO reported to the Washington VAAC that three exhalations at 0419, 0421, and 0500 on 21 November sent ash to 5-7 km a.s.l. A pilot report and information obtained from GOES-8 imagery revealed that an eruption at 1630 on 22 November produced an ash cloud that reached between 5.5 and 7.6 km a.s.l. Subsequent imagery through 1945 that day showed that there were two ash clouds from the eruption; one rose to ~5.8 km a.s.l., and the other rose to ~7.6 km a.s.l. On 27 November eruptions at 0330 and 1815 sent ash to 6-7 km a.s.l. A pilot report stated that a small eruption occurred sometime prior to 0700 on 28 November, sending ash to ~7 km a.s.l.

Volcanic activity continued at a high rate at Popocatepetl, with several small-to-moderate exhalations. The Mexico City MWO and the Washington VAAC reported that at 0947 on 14 November a small ash-and-steam exhalation produced a cloud that was visible on GOES-8 imagery; it reached an altitude of ~8 km a.s.l. The cloud rapidly dissipated as it moved briefly to the NNE. At 0910 on 17 November a steam-and-possible-ash emission produced a cloud that reached up to 6.5 km a.s.l. and was blown to the NNW. The Popocatepetl camera recorded an ash cloud from a steam-and-ash exhalation that occurred at 0730 on 20 November. The cloud reached ~6 km a.s.l., was blown to the NNW, and deposited light ash in the town of San Pedro Nexapa ~10 km to the NW of the summit. On 21 November three moderate ash-and-steam exhalations sent ash to between 6 and 7 km a.s.l. The volcano's alert level remained at Yellow Phase III.

During the week volcanic activity was high at Popocatepetl, with several exhalations and eruptions. CENAPRED reported that exhalations at 1456 and 1541 on 7 November sent ash clouds to 2 and 4.5 km above the volcano, respectively. The Mexico City MWO reported to the Washington VAAC that an ash-and-steam exhalation at 1150 on 9 November sent ash to ~9.5 km a.s.l. According to a Reuters article, CENAPRED stated that light ashfall occurred in Santiago Xalitzintla, the closest village to the crater. The Washington VAAC reported that on 11 November eruptions at 0739, 0818, 0845, and 1418 sent ash to a maximum altitude of ~9.5 km a.s.l. The volcano's Alert Level remained at Yellow Phase III.

The Washington VAAC, Mexico MWO, and CENAPRED reported that a series of small eruptions occurred during 1715-1806 on 1 November. GOES-8 imagery showed that ash rose to between 6 and 7 km a.s.l. and extended to the NNE. The series of eruptions lasted ~30 minutes and ash fall was reported in San Pedro Nexapa and Amecameca, which is ~20 km NW of the volcano. A Reuters article reported that on 4 November the increased level of volcanism caused Mexican authorities to carefully watch for signs of a strong eruption. CENAPRED increased the Alert Level at the volcano from Yellow phase two to Yellow phase three, which expanded the high-risk zone around the volcano from 7 to 10 km. At 2048 on 6 November an eruption sent an ash cloud to an altitude of 7.5-8.5 km a.s.l. It drifted towards the N and was followed by another eruption at 2130. Ash from both eruptions fell in the town of Santiago Xalitzinta. As of 7 November the Alert Level at the volcano remained at Yellow phase three.

Information is preliminary and subject to change. All times are local (unless otherwise noted)

"Members of the Mountain Climbers Group of the National Univ of México decended into the summit crater (elliptical shaped, [658 x 440] m) 25-26 January. The lower part of the main crater rim was 5,250 m above sea level and the crater was some 250 m deep. On its E floor there was a smaller circular crater about 200 m in diameter and 50 m deep with a small, roughly circular lake about 40 m in diameter and 10 m deep. The water was warm (29°C) and very clear with a greenish color. This small lake is not permanent and its dimensions are season-dependent. The main crater floor appeared somewhat changed since a previous visit in 1978, when the rim of the interior crater was at a lower level. On the bottom of both the main and inner craters were many small fumaroles with sulfur deposits around them. The temperatures of three fumaroles sampled in the bottom of the inner crater were in the range 97-99°C. In the E inner wall of the small crater were several fumaroles with vents that appeared to have greater activity. Their inaccessibility made it impossible to measure their temperatures but during the night they had a reddish glow, suggesting much higher temperatures. This kind of glow has not been reported inside the crater since 1938."

During 1986-87, a seasonal, nearly circular lake occasionally occupied the summit crater. The lake's pH was 2-2.7 and the temperature was 30°C. Continuous fumarolic activity began in August 1988. A March 1989 summit visit by Alejandro Rivera Domínguez revealed large sulfur deposits in the main and inner craters. New fumaroles (not observed in 1987-88) on the main crater wall emitted high-pressure sulfurous gas and steam to 300 m. No significant microseismicity or tilt was detected.

The Grupo de Montañismo y Exploración de la UNAM, led by Prof. José Manuel Casanova Becerra, climbed the volcano on 9 April. More than 20 new fumaroles were observed on the outer S flank about 200 m below the crater rim. These vents (up to 1 m in diameter) were not observed when the group visited the area 2 years ago. Steam columns reached 20 m height and there was a mild sulfur odor. The steam's temperature was probably near the boiling point (at about 5,100 m altitude). The average altitude of the crater rim was 5,300 m with the crater bottom 340 m below. Increased steaming (common during the season) was observed inside the crater.

One seismograph is sited near the volcano . . . . Researchers hope to build an observatory 12 km from the volcano with telemetric data capture. Current monitoring is from the Meteorological Observatory, Geophysics Dept, Univ Autónoma de Puebla, and from Yancuitlalpan Village, S of the volcano.

Most nearby earthquakes tectonic, but a few small B-type events detected

Summit visits in March and April 1989 revealed large sulfur deposits in the main and inner craters, and more than 20 fumaroles on the upper S flank that had not been present 2 years earlier (14:04). The following is a report from Servando de la Cruz-Reyna.

"On 15 November 1989, the first telemetering seismic monitoring station at Popocatépetl began operation near Tlamacas, on the NW flank at 3,980 m above sea level. The station consists of a 3-component, 1-second seismometer, continuously transmitting analog signals to the Instituto de Geofísica, UNAM, in México City. The vertical component is registered on a drum analog recorder, and all three components are digitally recorded on a hard disc, then on an optical disc, when signal amplitudes exceed a preset level. The transducer and recording devices are property of the National Seismological Service, UNAM; the transmitter/receiver set was donated by the Japan International Cooperation Agency. A basic 6-point deformation network was also set up in November 1989 on the NW flank between 3,980 and 4,525 m altitude.

"So far, the 9-month seismic record indicates that background seismicity is dominated by small regional earthquakes occurring within a radius of 40 km, mainly W and SW of the volcano. However, a number of very small local B-type earthquakes have been persistently recorded. The activity has not shown any significant changes. Though minor, the B-type activity and the persistent condensation of sulfur in the inner parts of the crater call for more intensive monitoring."

Increased fumarolic activity was observed from 4,500 m elevation during routine deformation studies on 25 February. A dilute white column, roughly the diameter of the summit crater, could be seen gently rising from the crater. The column was deflected by the wind immediately above the crater, even at low wind levels, and occasionally descended the flanks of the volcano, mainly to the NE. Snow on the NE crater rim was somewhat darkened, probably due to a wetting effect of the plume. The plume appeared to be mostly water vapor, with an occasional smell of sulfur. Tourists that have climbed to the summit reported that the crater is filled with a fog-like cloud, and there is a very strong smell of sulfur. The telemetered seismic station located at Tlamacas (4,000 m elevation on the NW flank) has recorded only normal background levels of seismicity, similar to those observed during the past three years. Based on current information, geologists believe that the increased fumarolic activity may be related to increased rainfall feeding the hydrothermal system. Installation of a third telemetered seismic station will be completed in the next few months. No change in fumarole activity was observed during fieldwork on 11 March, and seismic activity was low and within background levels.

Since January 1993 there have been increased reports of plumes, obvious in the afternoons, from residents of Puebla (45 km E, population 1.75 million). Geologists also reported increased fumarolic activity in February and March (18:02). Five observation posts have been installed in the vicinity of the volcano to monitor pH and temperature of water sources, and report plumes. The pH of the water has been consistent at 6.9, and the temperature is 17°C. A small circular lake (50 m diameter, 8 m deep) in the inner crater has increased in temperature from 29°C in 1986 to 51°C in March 1993. The primary sources of intense steam activity are at the bottoms of both the main and inner craters. Sulfur spots and deposits have been observed around the craters. At least 30 new vents on the floor of the main crater are releasing steam. Steam vents had not been observed since fieldwork in 1986. Daily visual monitoring is conducted from the meteorological station at Puebla Univ. No significant seismicity has been detected by the portable MQ-800 seismometer installed N of the volcano.

At least 15 scientific expeditions visited the summit in 1992 to investigate the eruptive history of Popocatéptl and prepare volcanic hazard maps. Seasonal activity, in the form of solfataric clouds from the inner crater, has been observed every year since 1986. From March-October 1992 a dense, greenish plume was observed, and a strong sulfur smell was detected in San Pedro (11 km SE) and La Magdalena (13 km SSE). A condensate of plume vapor had a pH of 3; water from the seasonal inner crater lake had a pH of 1.5. This type of plume was last reported during the 1943 eruption.

The following is a report by Hugo Delgado Granados of a visit to the crater 3 January 1993. Delgado has ascended the summit of Popocatépetl more than 50 times in the last two decades.

"Popocatépetl was climbed after several witnesses from towns around the volcano reported an increase of fumarolic activity from the crater. The ascent was made in clear weather, with the summit reached by noon. Fumaroles are much larger and in greater concentrations than on previous visits. Emissions are now visible from Tlamacas (4 km NW at 4,000 m elevation), as well as from Puebla (50 km E) and Cuautla (40 km S), depending on atmospheric conditions. During the last 10 years, slightly whitish emissions emerging through the lower crater rim to the NE were commonly observed in the morning. However, recent emissions have risen higher and are very dense with a yellow-green color.

"The crater is elliptical-cylindrical in shape, with a major axis of 850 m and a minor axis of 650 m (figure 1). There is a 460 m elevation difference between the main summit and the bottom of the crater (230 m below the lower crater rim). The shortest way into the crater is to rappel from a 70 m overhang on the lower crater rim; the remaining 160 m consists of a steep rocky slope that can be walked down. In the NE part of the main crater are the remnants of a former lava dome, 195 m in diameter and 50-70 m high, which was partly destroyed during the 1919-27 eruption, leaving a smaller inner crater. That crater contains an intermittent crater lake, acidic and greenish in color, at 4,955 m elevation.

Figure 1. Topographic map of the summit area and crater of Popocatépetl, showing the crater rim and locations of fumaroles in 1983, 1986, and 1993. Courtesy of Hugo Delgado Granados, UNAM.

"The locations of fumarolic vents observed in 1983, 1986, and 1993 are shown in figure 1. It is evident that the vents observed in 1993 are more numerous than in previous years. Gas emissions in 1983 and 1986 were whitish, suggesting a predominance of water vapor, and the plumes rose slowly. The 1993 fumarole emissions are more powerful (noises can be heard from the crater rim), denser, and more yellowish-green than before. Dense plumes from these fumaroles rise to the crater rim before being dispersed by the wind. Additional evidence of a higher sulfur content in the fumarole emissions is the widespread yellow coloration of snow around the crater rim and zones where sulfur is being precipitated on the crater floor."

During October-November a cluster of steam vents in Popocatépetl's 850 x 650-m summit crater produced substantial output. The resulting clouds reached 6,000 m elevation and were visible from 50 km E (Puebla City). On 13, 14, and 21 November, fumarolic activity led to 50-km-long, off-white plumes. The increased steam output has not been correlated to changes in visible structure, to seasonal conditions, or to seismic activity monitored on a portable seismometer. An unconfirmed account by one climber returning from the summit crater in mid-November told of strong sulfur smells, a small green lake, and occasional phreatic eruptions reaching over 200-m height.

In 1993 steam vents in the summit crater grew in number and output, leading to off-white plumes that extended 50 km downwind in mid-November (18:6 & 11). A visit to the summit crater on 30 January revealed conditions similar to those in June 1993, but the small intermittent lake had changed from clear green to a milky greenish color. This change was confirmed by photographic comparisons, and is thought to reflect the increased absorption of acid gases and steam.

SO2 output was measured by ultraviolet absorption correlation spectrometry (COSPEC) from a fixed-wing airplane on 1 February 1994. Wind speed, an important parameter in the SO2 flux calculation, was estimated at 36 km/hr by two means: 1) knowledge of the wind speed at the crater rim on the previous day, and 2) aircraft navigation instrumentation (Global Positioning System).

Fischer, Williams, Delgado Granados, and Siebe described their data and its significance. "Nine traverses of the plume were recorded and measurements averaged 1,200 ± 400 metric tons/ day SO2. The weather was good and the plume could be observed very well, favoring the quality of the measurements. This level of SO2 emissions puts Popocatépetl into a small league of half a dozen volcanoes worldwide that emit such high levels of magmatic gases. It is a symptom of reawakening and is usually associated with significant danger of magmatic activity. However, with no previous records from Popocatépetl, one must be careful about reaching conclusions before a good baseline is available."

Seismic events at the volcano are defined by Guillermo González-Pomposo, Valdés González, and Martin del Pozzo as follows. Type-A events have frequencies above 5 Hz, with identifiable and impulsive P and S phases, and S minus P times of 1.5 to 3 seconds. Coda duration (the "tail" of the event) is <40 seconds. Type-B events have frequencies in the 1.0 to 1.6 Hz range. Type-B events characteristically exhibit an emergent P phase arrival, that is the P phase increases in amplitude with time. They also have an S phase arrival that is hard to identify. Coda duration is <125 seconds. Type-AB events are made up of both low and high frequencies. The high frequency phases include impulsive P and S waves. These high frequency phases arrive first and extend for <5.5 seconds; the S - P times are 1.5 to 2 seconds. The amplitudes of P waves and S waves for type AB are smaller than for type A. The characteristic period of the low-frequency phases is 0.8 seconds, and these signals may last for 150 seconds.

During December type-A events took place 12 times, type-B events 62 times, and type-AB events 12 times. For each type, the frequency of events was roughly evenly distributed across the month. For type A there were 5 events on 8 December and 1-2 events on 4 other days. For type B there were 7 events/day on 3 December and 0-5 events on other days. For type AB there were 3 events on 15 December and 0-2 events on other days.

Seismic events for January and February are summarized on figure 2. During January there were 12 A-type, 12 AB-type, and 62 B-type events. Type-A events have frequencies above 5 Hz, B-type have frequencies 1.0-1.6 Hz, AB-type have both high and low frequencies (19:01). Corresponding totals for February were similar: 14, 5, and 62. During February the amplitude of type AB events increased by about 40%.

Figure 2. Popocatépetl seismicity for January-February 1994. Activity was monitored using the PPM seismic station located on the N flank at 3,900 m elevation. This station is part of the Mexican National Seismic Network. Courtesy of Guillermo González-Pomposo and Carlos Valdés-González.

Seismicity during March and April consisted primarily of B-type events. During March, 99 B-type events were recorded, an increase from the 62 recorded in both January and February (19:02). On two days (6 and 22 March) as many as 7 B-type events were recorded, and during a 3-day period (26-28 March) 16 events were registered. A- and AB-type events (11 and 6, respectively, during the month) were also recorded during periods of increased B-type activity. The number of B-type events continued to increase in April, reaching a total of 164 during the month. From 16 to 19 April, 33 events were recorded. The highest daily total was on 29 April, when 13 events were registered. Except for two A-type events on 2 April, no A- or AB-type events were detected. The seismic station, part of the Mexican National Seismic Network, is located at 3,900 m elevation on the N flank.

A new series of ultraviolet absorption correlation spectrometry (COSPEC) measurements were made by Univ de Colima scientists on 4-5 May from a Mexican Navy airplane. The measurements were requested by the Secretaria de Gobernacion through the Centro Nacional para la Prevencion de Desastres (CENAPRED). Between 1151 and 1359 on 4 May, the plume was traversed 20 times at an altitude of 3,900-4,000 m in partially cloudy to overcast conditions. Another 11 traverses were made at 3,950 m altitude between 0936 and 1148 the next day. The weather was again partially cloudy to overcast, and rainfall was detected upon returning. The aircraft's global positioning system (GPS) computed the wind speed independently for each traverse. These measurements were each used to make individual SO2 flux calculations, removing the need to calculate an SO2 estimate based on an average wind speed. This procedure is advantageous when the wind speed varies significantly. A statistical analysis of the time series was also performed.

The SO2 flux on 4 May ranged from 485 to 1,462 metric tons/day (t/d), with a standard deviation of 232 t/d and an average value of 900 t/d. This result is close to the value of 1,200 ± 400 t/d measured on 1 February (19:01). SO2 values dropped the next day to a range of 386-684, with a standard deviation of 89 and an average of 502 t/d. The 5 May results are not thought to be representative of the actual emissions because weather conditions were much more humid, resulting in a more effective gas-to-particle conversion that produced more H2SO4 aerosol droplets. An adequate baseline reference value for SO2 output has not yet been established, so any interpretations are preliminary and should be made with caution. A regular monthly schedule for additional COSPEC measurements is currently planned.

On 30 January 1994, Delgado, Siebe, and Tobias Fischer (Arizona State Univ) installed sampling boxes near the crater rim to monitor gas emissions. These boxes hold an open container of 500 ml 25% KOH solution to absorb acidic gases and allow measurement of variations in S/Cl/CO2 ratios (Noguchi and Kamiya, 1963). Since then, visits to the crater have taken place every three weeks. The following is an account of observations made during those visits through mid-May.

Intense fumarolic activity has been observed in the crater, in and around the inner dome, and from the crater walls. Sulfur deposits around the fumarolic vent glowed red during the day. Intense reddish glow at night was more common, although dense emissions often hampered observations. Hissing noises from several vents were due to powerful gas emissions that rose up to 1,000 m before being blown downwind. Most of the fumarolic column was bent by the wind to the E, NE, and SE during January-April. A tall, long plume from the volcano could be observed at distances of 40-60 km. The most dense fumarolic emissions came from the crater floor. Several small fumarolic vents associated with fractures in the crater walls and near the rim produced gas emissions, these were smelled by mountain climbers visiting the summit, and they contaminated the surrounding ice cap with sulfur. Most of the fumaroles were on the inner part of the crater rim, but some diffuse vents were observed on the outer SE and E flanks as low as ~5,000 m elevation.

During the 19 February crater visit, the small milky-green lake nested in the central dome had a temperature of 65°C and a pH of 1.5 ± 0.5. This crater lake description contrasts with observations from 1986 in color (very clear with a greenish tint), temperature (29°C), and pH (6.5) (11:01). Temperatures at fumarolic vents, measured using a thermocouple, ranged from 250 to 380°C. Fumaroles surrounded by the intensely glowing red sulfur deposits were inaccessible. The measured fumaroles were the same as those with temperatures of 97-99°C in 1986. Since late March-early April, additional water introduced to the system by precipitation has increased the vapor phase, causing the plumes to look more dense and whitish when emitted, but more diffuse when dispersed out of the crater. If no wind was present at 5,000-6,000 m altitude to disperse the plume, there was the appearance of a dense, dirty cloud near the volcano.

Increased fumarolic activity has caused alarm in surrounding towns and villages. Concern about future eruptions has prompted many people to climb the summit for observation of the activity. This has led to a proliferation of reports by untrained people describing "molten lava" (overheated sulfur) and "phreatic eruptions" inside the crater (reported as unconfirmed in 18:11). On 29 April a commercial airline pilot reported an "ash cloud" at ~5,800 m altitude 35 km SE of México City. No ash cloud was seen the following day during a routine visit to refill the KOH solution in the boxes. Careful observations inside the crater and on the ice around the crater did not reveal any recent ash emissions. The pilot very likely observed a fumarolic cloud, but the report of ash caused alarm among air-traffic authorities in México City.

Popocatépetl rises to 5,420 m above the Mexico and Puebla valleys, basins with >20 million inhabitants. The last significant eruptive period at this stratovolcano was in 1920-22, with fumarolic activity through 1927; minor ash explosions were reported in 1933, 1943, and 1947.

Following destruction of an ancestral volcano by a Bezymianny-type eruption during which a debris avalanche formed a 6.5 x 11 km caldera, the modern cone was constructed in two stages. The El Fraile volcano, formed prior to 10,000 years BP, was partly destroyed by later explosive activity. The current summit of Popocatépetl was formed to the south of El Fraile by repeated lava effusions until about 1,200 years BP. About 25-30 eruptions have occurred in the last 600 years, most of them apparently consisting of weak summit explosions.

Seismicity began increasing in March, with 99 B-type events, compared to December 1993-February 1994 when 62 B-type events were recorded each month (19:1, 2, & 4). This increase continued in April and May, with 164 and 295 B-type events, respectively (figure 3). However, after a peak of 16 events on 24 May, B-type seismicity began decreasing again through June (169 events). In general, when B-type events showed sharp decreases, A-type and AB-type events appeared (1-2 events/day). Throughout May and June only 7 A-type and 5 AB-type events were detected.

Figure 3. Daily number of B-type seismic events at Popocatépetl, January-August 1994. Activity was monitored using the PPM station located on the N flank at 3,900 m elevation. Courtesy of Guillermo González-Pomposo and Carlos Valdés-González.

As usual, seismicity during July and August consisted primarily of B-type events (figure 3). During these two months, B-type events were recorded more frequently than during much of January-March and less frequently than during much of May. Type-A, -AB, and -B seismic events at Popocatépetl were defined in 19:1.

Guillermo González-Pomposo and Carlos Valdés-González noted that when B-type seismicity increased in July and August, A- and AB-type seismicity declined. Both A- and AB-type seismicity remained at 0-1 events/day for July-August, except for two days when one or the other type reached 2 events/day. Overall, during July type-A events took place 4 times, type-B events 150 times, and type-AB events 6 times. During August type-A events took place 5 times, type-B events 165 times, and type-AB events 6 times.

In contrast with the moderate levels of seismicity seen in July and August, early July ultraviolet absorption correlation spectrometry (COSPEC) measurements made by ASU and UNAM researchers indicated a prodigious SO2 flux: a minimum of 575 metric tons/day (t/d) and an estimated "true flux" of 2,700-3,500 t/d. Their report on the 5,420-m-high volcano follows.

"We were able to make a driven traverse [using an automobile] of the plume of Popocatépetl on 1 July, 1994. The data showed an SO2 flux of 575 t/d, if a standard wind velocity of 1 m/s was assumed; this must be considered as the absolute minimum. Our best estimate of the true wind velocity was based on the National Airport measurements at 5 km above sea level (on 2 July) of 5 m/s. Therefore, our best estimate of the true flux was 2,900 t/d. An estimate of the uncertainty in this flux is complicated by measurements made on 2 July using the Trimble GPS (Global Positioning System) instrument on board the chartered aircraft. One aircraft traverse, at the crater level, suggested a wind velocity of ~30 m/s. So, we are reporting what seems to be a minimum realistic SO2 flux.

"At ~0900 on 1 July, the sky was relatively clear and the plume was visibly blowing to the SW. It appeared to rise a few hundred meters above the crater, before being blown by the wind. The white, cloudy plume remained visible for tens of kilometers, perhaps a hundred kilometers. By the time we were on the road that passes around the W margin of the base of Popocatépetl and Iztaccíhuatl, the cloud cover became sufficient to block any certain view of the plume. At 1700 in the afternoon, however, we were in the saddle between Popocatépetl and Iztaccíhuatl and had another very clear view of the plume. Its appearance then was similar to the way it had looked before, suggesting approximate stability for intervals of hours and days. Using the standard approach (Stoiber and others, 1983), we mounted the COSPEC on the passenger seat of the van, with the telescope looking vertically, and drove at roughly constant velocity (~30-40 km/hr). The traverse was more than 40 km in total length, with its center being at a point approximately straight W of the volcano's crater. Good maps facilitated geometrical corrections to allow for portions of the traverse not normal to the plume's axis.

"The airplane traverses made on 2 July used a plane flown by Sergio Zambrano who used his on-board GPS instrument to minimize all of the usual uncertainties concerning location, aircraft velocity, length of traverse, and angle between the traverse and the plume axis. However, the one measurement that we did not recognize adequately while airborne was the ability to realistically estimate wind velocity at the elevation of the plume, as it was dispersed. The five traverses gave extremely repeatable graphs [on the strip chart records] and the estimated flux was 3,100 ± 400 t/d (using the 5 m/s wind velocity measurement from the National Airport). Because we failed to recognize the possibility of using the GPS instrument for measuring the wind velocity we cannot accept the one [~30 km/hr] measurement as well constrained. If it were true, then the SO2 flux was enormous.

"Our measurements of SO2, by two different COSPEC methods on two different days, were remarkably similar. The plume looked very homogeneous, when we were able to see it on these two days. The increase in SO2 flux since measured by T. Fisher and others by aircraft on 1 February 1994 (1,200 ± 400 t/d) is very difficult to escape [19:1]. An increased gas flux is also consistent with the visual impression of H. Delgado upon climbing to the crater rim in August, that the gas emissions were greater with more loud sounds from the fumaroles within the crater."

Although the reported SO2 flux is strikingly large for a volcano not in eruption, it is too small to confirm with the satellite-borne TOMS, which detects masses of SO2 greater than about 5 kilotons (Bluth and others, 1992). Popocatépetl looms over the México and Puebla valleys, potentially threatening over 20 million people.

During late-October, Carlos Valdéz-González and co-workers identified a sudden, prominent (roughly 1.6- to 10-fold) increase in daily earthquakes compared to previous months (figure 4). Station locations and the terms "A-", "B-", and "AB-type" were previously defined (19:1-2). Although Figure 4 shows only B-type events, the other two types remained at 0-1 events/day during September and October. Prior to mid-October, the daily count of B-type events generally remained below 10, but by 28 October they climbed to 26. The B-type events for the first half of 1994 were previously published (19:06). Carlos Valdés-González noted that this was the fastest rate of increase in the last 23 months.

"A new series of ultraviolet absorption correlation spectrometry (COSPEC) measurements was made by scientists from Univ de Colima (A. González, J.C. Gavilanes and C. Navarro), UNAM (H. Hidalgo) and USGS (T. Casadevall) on 5 November from a rented Cessna 310 airplane. The measurements were requested by the Secretaría de Gobernación through the Centro Nacional para la Prevención de Desastres (CENAPRED). Between 1024 and 1148 on 5 November, the plume was traversed 12 times at an altitude between 3,539 and 4,545 m a.s.l. [above sea level] in partially cloudy conditions. The aircraft's global positioning system (GPS) computed the wind speed independently for each traverse. These measurements were each used to make individual SO2 flux calculations, removing the need to use average wind speed (19:08). This procedure is advantageous when the wind speed varies significantly. SO2 data were sent to a datalogger, besides the typical COSPEC strip chart. All the recorded data were transferred into a personal computer where evaluation software produced the final SO2 results together with a statistical analysis of the time series. A manual SO2 determination using data from strip chart records (as reported in 19:08) was also made by C. Navarro; it reproduced the average values within 2.4% on average.

"The SO2 flux on 5 November ranged from 924 to 1,877 metric tons/day (t/d), with a standard deviation of 285 t/d and an average value of 1,261 t/d. Table 1 compares our recent measurements with those of 4 May, which were determined with the same methodology (19:04). The SO2 flux increased substantially between 4 May and 5 November. Although our determinations show absolute values less than those reported by other authors (19:1 & 8), both data sets show increased SO2 flux."

Table 1. Popocatépetl SO2 flux measurements on 4 May and 5 November 1994. Courtesy of Ignacio Galindo, Univ de Colima.

A new episode of explosive activity began at Popocatépetl volcano on 21 December 1994 (figure 5). The eruption followed increases in seismicity, SO2 flux, and fumarolic activity seen during the last 13 months. Although in the last year seismicity rose and fell several times, during late-October there was a sudden, prominent (roughly 1.6- to 10-fold) increase in daily earthquakes compared to previous months. Measurements of the volcano's total SO2 flux were consistently large (some airborne measurements averaged over 1,000 tons/day). During October-November 1993 a cluster of steam vents in the summit crater produced clouds that reached 6,000 m elevation, several-hundred meters above the 5,465 m summit. These clouds sometimes stretched for 50 km.

Figure 5. Base map of Popocatépetl and vicinity (elevations taken from the 1986 México City 1:250,000 topographic sheet).

Eruptive activity. Near midnight on 22 December 1994, Servando De la Cruz sent the following report.

"The fumarolic activity that has been developing during the last two years or so culminated on early 21 December 1994, when a series of volcanic earthquakes, probably associated with phreatic explosions, marked the beginning of a new stage of eruptive activity. The seismic events, detected at 0131, 0132, 0138, 0140, and 0148, were very impulsive, high-frequency, short-duration signals, and were followed by a major, lower-frequency event at 0153. The events were recorded by four telemetric stations within 11 km of the volcano operated jointly by CENAPRED and the Institutes of Geophysics and Engineering of UNAM. As the day cleared an ash plume was observed for the first time in decades emerging from the volcano's crater. The ash emission was moderate and produced an almost horizontal plume causing a light ashfall over the city of Puebla, about 45 km ENE of the volcano's summit. A helicopter flight at 1030 showed that most of the ash issued from near the lower NE rim of the inclined crater. A radial fissure on the NE flank of the cone displayed some steam-producing vents, though the cloudy conditions make this interpretation equivocal. Old cracks in the glacier appeared to have extended a significant amount towards the W. A second flight at 1430 the same day revealed a substantial increase in ash production (about 3-4 times the amount observed in the morning). The light-gray ash appeared to be emitted episodically, with "puffs" every few minutes.

"The seismicity consisted of mostly low-amplitude B-type earthquakes and concurrent high-frequency A-type events. Though this seismicity remained lower than during night of 21 December, during the next day the seismicity again increased. At this stage and after several consultations between the scientific group and the Civil Protection authorities, an evacuation of the 19 most vulnerable towns and villages on the E sector of the volcano was started around 2100 of 21 December, and about 31,000 persons were moved during the night to shelters in safer areas. Since then the situation has remained fairly stable, though long-duration, low-amplitude tremors appeared in the night of 21-22 December, and continue."

Claus Siebe reported that climbers at Popocatépetl reached the summit, which lies along the W margin of the gaping summit crater's rim, both on the day before the eruption, and hours after the 21 December eruption started. On the day before the eruption visiting climbers could see the crater lake and sparse fumaroles. They reportedly heard no hissing sounds and they smelled less odor from sulfur-bearing gases than in previous months.

Curiously, the six volcanic earthquakes that took place between 0130 and 0200 on 21 December were not felt, and the presumably associated phreatic summit explosions were not heard by any of about 25 mountain climbers at Tlamacas, 4 km N of the summit (figure 6). The climbers, who said they started ascending the mountain around 0400 on 21 December, did not notice anything unusual until they neared the crater rim. Just prior to reaching the rim, a few minutes before 0800, climbers were stunned by what they thought was the sound of jet engines. At the crater rim they saw new bombs as large as 40 cm that had been thrown out of the 250-m-deep crater and had burrowed deep impact-pits in the snow. According to Siebe: "Most climbers who reached the summit that morning thought that the activity was normal, because they had never visited Popocatépetl before." At the summit, the climbers said they could not see the crater floor even though a strong wind was blowing. They descended back down the mountain without incident.

Siebe was at Tlamacas at 0900 on 21 December during clear weather. He observed a continuous ash plume rising 100-500 m above the crater with pulses at intervals of 1-5 minutes. The plume was carried at least 60 km E. Enough silt- and sand-sized material reached Puebla to produce a thin coating on cars. The ejecta appeared to be non-juvenile, and it contained pyrite, sulfur, and Ca-sulfate.

A report from Steve McNutt indicated that the volcano began to quiet down on the afternoon of 25 December. During the night of 27-28 December a M 2 earthquake took place; for reference the largest prior event in the recent past was M 2.9. On 27 December tremor was barely perceptible and a few small low-frequency events took place. During the 24-hour period ending about midday on 28 December there were ~30 low-frequency events. Tremor roughly doubled between 23 and 24 December, but then during 25-28 December it dropped and became barely detectible. No specific seismic data were available for dates after that, though seismicity did increase again and an audible explosion was heard roughly 10 km from the summit at about 1300 on 31 December. Investigators planned to install about four new seismic stations to improve spatial and azimuthal coverage, and to add one station close in.

By 27 December all but three of the previously evacuated towns had been reoccupied; those towns not reoccupied were subject to lahar hazard. A glaciologist made an initial helicopter inspection of the glacier looking especially for signs of abnormal melting. No report was available at the time of this publication, but steps to monitor the glacier included both a daily inspection flight and a video camera aimed at it from 5 km away. The last of the three previously evacuated towns was reoccupied by 28 December.

News reports. A 21 December Associated Press story said Popocatépetl, "spewed a column of roiling black ash Wednesday, dusting villages and farmland but causing no injuries" and that "television footage from traffic helicopters showed a dense column of ash belching from the summit."

As of 23 December, an Associated Press report noted that the Puebla state government said 75,000 people would be evacuated from the countryside around the volcano. Some other news reports put the number of evacuees at about 50,000. One of the evacuated towns, Santiago Xalitzintla, is located about 13 km NE of the summit. The town sits along the road over "Paso de Cortez," the pass between Popocatépetl and the adjacent Quaternary stratovolcano to the N, Iztaccihuatl (figure 6).

A 26 December United Press International news report noted that "Jorge Martinez Soto, a researcher at the Univ of Puebla, said the amount of smoke and ash being emitted from the volcano . . . diminished by about 75 percent since last week . . . ."

Plume imagery and transport modeling. Although the 21 December eruption plume may appear on satellite imagery, to our knowledge no investigator has yet announced having found it. There is an AVHRR (channel 1) image of a Popocatépetl plume on 22 December at 0818 (1418 GMT). That image shows a SE-directed plume tens of kilometers long. There are also three other AVHRR images for plumes on 26, 27, and 28 December. All four images are available via e-mail from Melissa Seymour. We learned of these images at press time and although we have not had time to see them first-hand and tabulate plume orientations, the imaged plumes reportedly trailed southward.

The Synoptic Analysis Branch (SAB) of NOAA/NESDIS first reported Popocatépetl activity at 1530 (2130 GMT) on 26 December for an eruption that took place at around 1300. A SIGMET (Significant Meteorological Event) notice was posted from México City announcing that a new eruption had taken place and that the plume from this eruption reached an altitude of about 6.7 km (22,000 feet). SAB later continued to describe the shape of the plume associated with this eruption based on GOES-7 and -8 data (table 2 and figure 6). A report later that day (26 December) indicated that the volcano had continued to erupt, creating a visible plume that at 1745 extended to 50 km E. At 0745 the next day (27 December), a GOES-8 visible satellite image of the plume suggested a gently curving, funnel-shaped mass tracking NE (figure 6). Based on the lack of infrared signatures and on their visible signatures, all the plumes reported in table 2 and figure 6 were thought to be of low density.

Table 2. Visible (GOES-7 and -8) satellite images reported for Popocatépetl. The time of initial eruption for all these plumes was around 1300 (1900 GMT) on 26 December. The third and fifth plumes listed are shown graphically on figure 6. Courtesy of SAB.

Figure 6. Popocatépetl ash plume at a) 0745 (1345 GMT) on 26 December 1994 (black) and b) 0815 (1415 GMT) on 28 December 1994 (stipple) as seen on satellite imagery. The northern edge of the longer plume just touched the Gulf Coast near Tampico. Courtesy of Nick Heffter.

A modeling program called "VAFTAD" was used to forecast the transport and dispersion of the plume from the 26 December eruption (see references and description of VAFTAD in the report for Rinjani, 19:06). VAFTAD produced a series of visual ash cloud forecasts such as those on figure 7, which showed the plume initially covering both quadrants in the E half of the volcano and then traveling NE along about the same path taken by actual plumes seen in the GOES imagery (table 2 and figure 6). The models forecasted that after about 24 hours the plume would travel NE over the Gulf of Mexico.

Figure 7. Examples of forecasts of the Popocatépetl plume after a large eruption. Both of these forecasts were for an initial erupted plume height of 7.6 km (25,000 feet) and an eruption duration of 24 hours. They both portray the elevation range from 6 to 10 km (20,000-35,000 feet). The forecasts were based on an eruption beginning at 1300 (1900 GMT) on 26 December. The map on the left shows the forecast plume 12 hours after the eruption began, the map on the right, 24 hours after the eruption began. Courtesy of Nick Heffter.

VAFTAD uses wind and pressure data updated twice daily on grids with spacings of 91 km in the USA and 1 degree over the rest of the globe. The model assumes the eruption delivers a mass load to the atmosphere. The mass load is not scaled to the actual mass of the eruption, but rather the load is assumed to be 1 gram (composed of spherical particles with a density of 2.5 x 106 grams/m-3 in a size range of 0.3-30 µm in diameter). VAFTAD computes transport and dispersion assuming particles are carried by advection both horizontally and vertically, diffuse with a bivariate normal distribution, and fall according to Stoke's law with a slip correction. Calculated ash concentrations have been correlated with satellite imagery for defining the visual ash cloud forecasts.

One noteworthy aspect of the Popocatépetl plumes is the relatively large height of the summit crater (elevation ~5,215 m). Even small, low-energy eruptions from this high altitude vent can erupt material to 6 km (~20,000 feet) elevation.

So in essence, these ash cloud forecasts serve best for hazards planning purposes. A key use, in fact, is to warn airline pilots of the airspace most likely to contain volcanic ash particles. Besides the other hazards discussed in Boudal and Robin (1989), a large eruption from Popocatépetl could affect air travel in routes over parts of NE México and much of the Gulf of Mexico.

Eruptive history. In the Holocene Popocatépetl has produced both effusive and pyroclastic activity. The latter has ranged from mild steam-and-ash emissions to Plinian eruptions accompanied by pyroclastic flows and surges. Vigorous Holocene explosive activity took place in three periods (in years before present, ybp): a) 10,000 to 8,000, b) 5,000 to 3,800, and c) 1,200 to present (Boudal and Robin, 1989). An effusive period from 3,800 to 1,200 ybp ended with a vigorous explosive eruption that both enlarged the summit crater and generated St. Vincent-type pyroclastic flows. Another large explosive eruption, about 1,000 ybp, produced pyroclastic flows that descended the N flank.

Historical eruptions depicted on Aztec codices date back to 1345 AD. About 30 eruptions have been reported since then, although documentation is poor. Most historical eruptions were apparently mild-to-moderate Vulcanian steam and ash emissions. Lava flows restricted to the summit area may also have occurred in historical time, but cannot be attributed to specific eruptions. Larger explosive eruptions, possibly Plinian in character, were recorded in 1519 and possibly 1663. The last significant activity took place from 1920-22. Then, intermittent explosive eruptions produced 6.6-km-tall columns and extruded a small lava plug onto the floor of the summit crater. Ash clouds were also reported in 1923-24, 1933, 1942-43, and 1947.

Popocatépetl erupted for the first time in decades on 21 December 1994. In the wake of the eruption, Claus Siebe contributed the following report.

"Visual observations at Popocatépetl since 21 January reveal that the emission of ash has decreased in general and become more irregular. Longer time intervals between single series of ash outbursts are observed. Outbursts of ash still occur in single pulses, normally several pulses in a row spaced at intervals of 2-5 minutes. During longer periods of low ash-emissions fumarolic gases are still emitted, preventing direct observation of the crater bottom. Despite this restricted visibility, it is certain that the ash emissions originate in the same area where previously a milky-green lake was located. This lake occupied the depression of the small interior crater that formed during the 1920-27 eruption, when a lava dome was emplaced. During that eruption the lava dome was destroyed by a Vulcanian explosion that created the depression. This interior crater is not located in the center of the main crater but has an eccentric position close to the eastern crater wall.

"In this respect, it is noteworthy that only one of the eight springs monitored by me during the past months has shown a conspicuous anomaly. This spring is located at Axocopan, near Atlixco, 25 km SE of the volcano. Measurements of pH on 11 July, 18 August, and 17 November fluctuated between 5.8 and 6.1. The last measurement taken prior to the 21 December eruption was on 10 December and yielded a pH of 2.5. This anomaly can be regarded as another premonitory sign. The last measurement was taken 14 January 1995 and yielded a value of 5.5, which is almost back to normal. Maybe, during the last weeks prior to the eruption enough pressure built up in the system to allow "magmatic juices" to be expelled laterally and affect the pH of the Axocopan spring."

Following the 21 December eruption a new series of ultraviolet absorption correlation spectrometry (COSPEC) measurements were made by scientists of the Univ of Colima (I. Galindo, A. González, R. Ayala, and L. S. Ivlev). The measurements were requested by the scientific committee organized through the auspices of the Centro Nacional de Prevención de Desastres (CENAPRED). The post-eruption measurements began 48 hours after the eruption, and were also made on 5 other days, with the latest reported being 14 January (figure 8 and table 3).

The COSPEC instrument was deployed from rented airplanes (flown by R. González and C. De la Cruz) in a variety of weather and atmospheric conditions including air with fine suspended ash (23 December), high overcast clouds (27 December), and clear sky. The plume top was situated at 4,300-5,740 m altitude and traverses beneath the plume were made at altitudes between 3,000 and 4,545 m. The plume was identified and two navigational bench marks were placed using the aircraft's global positioning system (GPS) to assure that the traverses were perpendicular to the plume's axis. The speed and direction for both the plane and the wind were computed at two points inside of the plume. In addition, at the end of some sessions a parallel flight above the plume used the GPS to resample the wind speed.

Figure 8 and table 3 suggest the SO2 flux baseline for 1994 was about 1,000 tons/day, rising to about 4,000 tons/day during the eruption on 24 December. The airborne observations also indicated that the plume did not reach more than ~500 m above the summit, meaning that the thermal output of the volcano was low. The plume mainly consisted of water vapor with a minor ash component. Its color was dark gray to light brown.

Two tiltmeters have now been installed within several kilometers of the summit, and during their first 10 days of operation in mid-January they remained stable ( ± 0.5 µrad). Reflectors for survey lines have been replaced on the mountain's accessible N side; and they have been recently surveyed at 5-7 day intervals. Reportedly, in mid-January the surveyed line-lengths have also remained stable. In addition to four or more seismometers on the mountain, a TV video camera with a telephoto lens was aimed at the summit crater, affording a view of the summit monitored from CENAPRED data center. John Ewert reported that in the first week of January banded tremor prevailed, accompanied by nearly constant ash emission. The seismicity has quieted since then. In mid-January scientists saw several small earthquakes that were followed about 90 seconds later by distinct visible "puffs" seen coming from the summit on the TV video monitor. This, and other behavior, were taken to suggest the magmatic system was open and under comparatively little pressure. A terse summary is available on both activity as late as the end of December, and recommendations for future monitoring (S. McNutt, 1994).

Increases in both the frequency and intensity of ash emissions was noticed by Claus Siebe on 12 and 13 February during fieldwork on the slopes of the volcano. Plumes observed were the highest since this eruption began on 21 December (19:11-12), with an estimated height of ~2,500 m reached on 13 February. Large puffs observed at 1525, 1630, and 1730 on 12 February did not rise very high because of a strong wind blowing towards the NE. On 13 February the wind was weaker, especially during the morning, and plumes occurred at about 0930, 1100, 1130, 1525, 1625, and 1950. In addition, anomalously acidic pH values were measured at Ojo de Carbon springs located on the S slopes near Tlapanala.

During the Holocene Popocatépetl has produced both effusive and pyroclastic activity. About 30 eruptions are known since 1345, although early documentation is poor. Most historical eruptions were apparently mild-to-moderate Vulcanian steam and ash emissions. Larger explosive eruptions were recorded in 1519 and possibly 1663. Activity in 1920-22 produced intermittent explosive eruptions and a small lava plug in the summit crater. Ash clouds were also reported in 1923-24, 1933, 1942-43, and 1947.

On the morning of 21 February at 1105, for the first time since eruptions began on 21 December 1994, Claus Siebe was able to look into the crater from a helicopter without fumes or ash impeding visibility. A small crater surrounded by a tuff cone composed of light-brown to gray silty-sandy ash occupied the site of the former lake. Judging from the color, he interpreted the loose ash to be mostly non-juvenile. A plume was emitted from a depression in the ash cone at 1115 and rose ~3 km above the crater rim. No snow has fallen in recent weeks, and all the snow and ice in the summit area was covered by a thin coat of ash.

. . . SO2 flux was estimated twice during January using COSPEC. On 15 January scientists made airborne measurements but were unable to establish a GPS navigational fix for 2-3 hours and so made wind speed estimates from map positions and estimates by their pilot, Sergio Zambrano. On 28 January the plume was traversed by a van on a route between the Puebla airport and a junction N of Atlixco; wind speed was from pilot reports to the Puebla airport. Two 15-minute eruptions of dark ash were noted (at 0922 and 1015). Results of these SO2 flux measurements were as follows: 1) 15 January, 3,680 ± 300 tons/day; 2) 28 January, 2,000 ± 1,000 tons/day.

At 1000 on 27 January a light beige plume rose no more than 100-200 m above the crater rim and was visible downwind for about 100 km. In addition, sufficient ash fell on the Puebla airport during the night of 27 January to make the tarmac (airport surface) light in color and to visibly cover freshly washed planes.

"We report on Popocatepetl seismic activity during the interval 21 December 1994 to 2 May 1995. Activity was monitored using seven seismic stations located around to the volcano above 2,600 m elevation (figure 9). These stations are part of the Popocatepetl Seismic Network. Beginning 21 December, the volcano changed dramatically in its seismic and fumarolic activity. Several explosions emitted ash that fell on Puebla City, an area located about 50 km away. About 22 hours after this activity, seismic tremor was observed for the first time at several stations.

Figure 9. Stations of the Popocatepetl Seismic Network (triangles) and epicenters for located events detected 21 December to 2 May 1995 (dots). Courtesy of Instituto de Geofisica, UNAM.

"In the 21 December-2 May interval we located 75 seismic events in the vicinity of the volcano (figure 9). We used arrival times from digital records from at least three stations and located the events using Hypocenter software. The average standard location errors in the horizontal and vertical directions do not exceed 1 km with a standard deviation of 0.14 km (figure 10). Earthquake magnitudes (calculated using a coda length magnitude for tectonic events in Mexico) ranged between 1.4 and 3.4 (as represented by different sized dots on figure 10). The E-W cross section of the hypocenters (figure 10) shows a concentration of seismic events in a circle of 3.0 km diameter and in a conduit that connects to the overlying crater. These results crudely suggest a magma chamber located below sea level and connected to the volcano crater. A N-S cross section suggests the same findings.

Figure 10. An E-W cross section of the hypocenters beneath Popocatepetl for the interval 21 December 1994 to 2 May 1995. Earthquake magnitudes are shown by dot sizes; the size of error bars are discussed in the text. Courtesy of Instituto de Geofisica, UNAM.

"During the first four days (21-24 December) seismic tremor was continuous and of high amplitude. During the following 20 days (25 December-13 January) tremor was also continuous, but the amplitude diminished five-fold compared to the first four days. After that, in the next 45 days (14 January-28 February), tremor turned sporadic with durations of about 10 minutes and with amplitudes comparable to those in the first four days. During the last 60 days, tremor became more sporadic with smaller durations, but it still had amplitudes similar to, and in some cases exceeding, those of the first four days.

"On 12 March an expedition lead by Enrique Chaves-Popuard reached the volcano's summit. The meteorological conditions allowed the team to videotape the interior of the crater. The following observations were made: a) the crater lake disappeared, b) three new craters appeared at the foot of the main crater's E wall, c) most of the fumarolic emissions came from these new craters, d) the number of small fumarolic vents has increased in the older inner crater, and e) several fumarolic vents were observed in the S and E walls of the main crater."

Several months of explosive activity began at Popocatépetl on 21 December 1994. The eruption followed over a year of increased seismicity, SO2 flux, and fumarolic activity.

Beginning at 1140 on 6 January 1996, observers in Puebla saw a steam column reaching ~6.1 km (20,000 ft). Meteorologists at the Synoptic Analysis Branch of NOAA were unable to see this steam column in either the GOES visible or infrared satellite imagery. On 7 January, as viewed from towns at the volcano s base (such as Amecameca and Atlautla), strong fumarolic activity continued from the crater.

Three days later, on 10 January, a helicopter from the Procuraduria General de la Republica flew L. Cardenas, H. Delgado, C. Siebe, and others over the volcano. They noted that fumaroles in the crater appeared rather weak, including those in the field on the volcano s NW side first noticed in April 1995. New fumaroles had sprung up on the N crater rim. No emanations were seen coming from the E-flank fumarolic field.

Later, when observed in conditions of good visibility from surrounding towns on 13-14 and 17-18 January, fumarolic activity was absent. Around this time a climb to the crater rim by Cardenas and Delgado enabled them to look inside; they again saw fumarolic activity that was weaker compared to that seen during most of 1994 and 1995. Despite this weak activity, two boccas on the rim of the old inner lava dome each contained an intensely hissing fumarole. The rocks cradling the fumarole glowed a reddish color visible in daylight, attesting to very high temperatures. In addition, fresh rockslide rubble that had probably sloughed off the crater s N walls lay on the crater floor. The crater s inner E wall looked extremely altered, suggesting that it may be susceptible to additional mass wasting in the near future.

They saw more intense fumarolic activity from the crater rim than they had ever seen before, possibly indicating that the main conduit below the crater floor was in the process of sealing. In this context they advised against mountaineers climbing to the summit area, because small Vulcanian explosions could occur at any time without warning.

C. Valdés-González, G. González-Pomposo, and A. Arciniega-Ceballos (UNAM) reported on seismic activity from November 1995 through early January 1996. Activity was monitored using seismic station PPM, a part of the Mexican National Seismic Network, located on the north flank at 3,900 m elevation. Seismic events were classified as type-A, -B, and -AB (BGVN 19:01).

Type-B events dominated during 1 November-8 January, an interval when 617 were recorded. Where data are available in the interval 1 November-8 January (figure 11), the number of type-B events ranged between 3 and 18 events/day. During this same interval, type-A and -AB events registered only 7 and 6 times, respectively. Fewer than 13 type-B events/day registered during late November through early January, ending on 5 and 6 January 17 and 18 events/day were recorded, respectively.

Figure 11. Type-B events (1.0-1.6 Hz) at Popocatépetl, 1 November 1995 to 8 January 1996. Blank where data were unavailable. Courtesy of UNAM.

Galindo and others (1995) summarized the available SO2 flux estimates for 2 January 1994-28 January 1995. Other reports in the same volume described different facets of the volcano s behavior, including those relevant to public health (e.g. ash- aerosol dispersion).

An ash-emission event was detected at 0349 on 5 March when continuous tremor began. This seismicity remained at relatively high levels for about one hour, and then decreased. Mild ashfalls were reported in the immediate area around the volcano, particularly in the N sector. During a helicopter reconnaissance flight at 1200, ash deposits were confirmed, especially in the close neigborhood of Tlamacas (figure 12). The glacier and snow near the summit were entirely covered by ash, confirming statements made by witnesses who saw ash emissions in the morning. A vigorous ash-and-gas column could be seen rising ~800 m vertically; it dispersed NE in a long plume. A sulfur smell could clearly be perceived near the crater. The emission of gas, steam, and ash appeared to come from the same three sources in the E side of the crater that produced the 1994-95 activity (BGVN 19:11, 19:12, and 20:01-20:04). This event on 5 March seemed very similar to that of 21 December 1994, but perhaps about an order of magnitude lower, an intensity comparable to the levels of activity observed on 26 December 1994. The only activity that may be regarded as precursory was a small A-type event at 0227 (M 1.2).

Figure 12. Base map of Popocatépetl and vicinity (elevations taken from the 1986 México City 1:250,000 topographic sheet).

Tremor activity slowly decreased through the night of 5-6 March. At 0710 on 6 March another sudden increase in the gas and ash emission rates was accompanied by tremor signals comparable to those of the previous day. These levels persisted through at least 1030 on 6 March. During a second helicopter reconnaissance flight, between 0825 and 0930, the ash plume was larger than the previous day, and directed E. However, the plume, consisting of steam, gases, and dilute fine-grained ash, bent as it exited the crater. Considering the low wind speed at the summit (~28 km/hour), this suggested a low thermal power in the emission.

At 1245 on 6 March a new and stronger ash-emission event was detected. Volcanic tremor increased correspondingly. Tremor amplitude continued to increase until 1532, when it reached the relatively high levels of early 5 March, where it remained for at least three hours. The ash plume, now with a higher particle density, was blown SE at ~22 km/hour. Besides tremor, during the first two days of the eruption there were mixed low-magnitude A- and B-type earthquakes at shallow to intermediate depths, with the greatest concentration at ~5-9 km beneath the summit (figure 13).

Figure 13. Cross-section of earthquake hypocenters at Popocatépetl during 5-7 March 1996. Triangles indicate the position of seismic stations. Bars indicate uncertainties in the location. All dates and times are local. Courtesy of Carlos Valdes-Gonzalez, UNAM.

Volcanic tremor amplitude and the ash emission rate remained fairly constant until 1030 on 7 March, when tremor amplitudes decreased by a factor of about two. However, a helicopter flight at 0800-0900 showed some increase in the apparent emission rate. Mild ashfall on the volcanoþs flank was observed under the plume; wind speed was low (10-12 km/h). These conditions remained stable until 7 March at 1650, when tremor amplitude and duration increased to levels exceeding those of December 1994. Stronger winds (60 km/hour at 1100) bent the plume horizontally from the crater, dispersing the ash farther E. Tiltmeters showed some oscillations, probably related to the high tremor level, but no actual deformation was detected. High tremor amplitudes persisted until 1400 on 10 March, when tremor amplitude and the ash emission rate slowly underwent a 5-fold decrease.

Low-level activity persisted until 11 March at 1800, with three important exceptions. At 1845 on 10 March, a strong emission produced an ash column nearly 3 km high accompanied by a small 2-minute duration B-type volcanic earthquake. These events repeated at 0921 and 0937 on 11 March. The 0921 event was preceded by a fairly high-frequency A-type earthquake at 0906. On 11 March at 1800, the pattern of activity started to return to continuous tremor and ash emission. These tremor signals have been interpreted as high-speed exhaust of volcanic gases that remobilize non-juvenile ash.

Satellite imagery, 10-11 March. Analysis of satellite imagery by the NOAA Synoptic Analysis Branch revealed that an eruption at 0245 on 10 March was followed 30 minutes later by a larger burst. The height of the ash was estimated to be just above the summit level (5.5 km). By 1315 that day the ash plume had extended S and SW as far as the Pacific Ocean (figure 14a). Movement of the ash cloud by 1515 suggested that the ash probably extended upwards to ~7 km. The last usable visible imagery on 10 March (at 1815) showed the thicker portion of the ash cloud over the ocean (figure 14b), but less ash in the immediate vicinity of the volcano. This correlates with the decrease in tremor amplitude and ash emission that began at 1400 as noted above. However, infrared imagery indicated continued eruptive activity through the night.

Figure 14. Sketches of the ash plumes from Popocatépetl based on visible satellite imagery, 11-12 March 1996. Solid areas are denser zones of the eruption cloud, lightly stippled areas are zones of the cloud with less ash. Note that scales vary. Courtesy of the NOAA Synoptic Analysis Branch.

The first visible imagery the next morning, at 0915, showed the volcano still erupting with the ash moving S and then W over the East Pacific Ocean (figure 14c), possibly with thinner ash even farther away. A stronger eruptive event at 0945 (probably the 0921 and 0937 events as noted above) sent a plume to perhaps 7.5 km altitude where it was blown SE. The cloud from these events had dissipated by the time of the next visible image at 1015. Eruptions became intermittent over the next three hours, with the estimated plume height remaining at ~7 km altitude. Ash seen on imagery at 1315 was present in a very narrow S-directed area (figure 14d), with thinner ash detected over the ocean.

Continued ash emissions; new lava dome and lava flows in summit crater

A new eruption began early on 5 March with continuous tremor followed by small ash emissions (BGVN 21:02). Low-level ash emissions continued through 11 March with some larger events on 10 and 11 March. Those episodes generated plumes that extended SW over the Pacific Ocean.

After 11 March and through the 19th, the overall level of activity appeared to have reached a steady state. Fumarolic activity alternated with 4-6 short-duration ash emissions each day from the same vents as the 1994-95 episode. These emissions formed short-lived ash columns that were carried away by the wind. Light ashfalls were reported from several towns around the volcano, particularly to the E and S. Seismicity, as low-level tremor accompanied by minor A- and B-type volcanic earthquakes, also showed almost stationary patterns and energy release rates. No deformation was detected by the 3-tiltmeter network on the N flank.

Satellite imagery during this interval revealed intermittent plumes extending E at altitudes around 6-7 km. Late on 13 March the plume was visible as far as 340 km ESE of the summit at 5-7 km altitude; ashfall was reported in Puebla, 70 km E. Large plumes of very thin dispersed ash blowing E over the Gulf of Mexico were observed through 15 March, with denser plumes closer to the volcano. During 15-19 March, when observed on satellite imagery, plumes averaged ~20 km wide and 60 km long before they dissipated; altitudes were in the 5-7 km range.

Ash emission increased between 2000 on 19 March and 0300 on 20 March, when characteristic signals of eight emission events or 'puffs' were detected by the seismic monitoring network. Afterwards, the emission-event rate returned to the previous range of 4-6 events/day. This, combined with stronger winds towards the E, produced light ashfalls on towns in that direction. The 'puff' events were detected on top of a moderate level of volcanic seismicity, consisting of A- and B-type events and low-level tremor, as well as strong signals from Pacific-coast tectonic earthquakes unrelated to the volcanic activity.

On 21 March the ash emission rate remained stable. The next day, the puffs' frequency increased to ~9/day, but their size decreased. Average height of the ash plumes was ~500 m above the summit, and duration <5 minutes. This activity continued without significant changes until 25 March, when the rate of ash emissions reached 8 puffs between 1030 and 1230 before returning to a rate of 8-10/day. This condition prevailed until 28 March, when another increase in the level of activity was detected similar to that on 25 March. The ash puffs were easily recognized in the seismograms as 30-40 seconds of tremor followed by an impulsive signal, similar to seismic events in the 1994-95 episode. Although the release of seismic energy increased after 25 March, the levels never reached high values, and remained well below the energy level of 5 March. Seismicity decreased again in late March.

Plumes after 20 March continued to be visible on satellite imagery, and were interpreted based on wind data to generally have been below 7 km altitude, with some slightly higher. However, poor weather and low levels of activity limited the number of plumes identified. Aviation notices from Mexico City and observers at the Puebla airport through 4 April continued to report ash at low levels, usually within ~20-30 km of the summit, blowing in easterly directions.

On 29 March during a COSPEC flight, Lucio Cardenas, Juan Jose Ramirez, and Hugo Delgado observed a new lava dome with an area of 400 m2 on the E side of the crater floor along the rim of the inner crater (a lava dome destroyed during the 1920-27 eruption). This new lava dome was observed coming from a source outside that inner crater but flowing into the it. Another helicopter flight later that day confirmed that block-lava was flowing from a vent located between the vents opened on December 1994 and the 1919 craterlet near the center of the crater. This lava slowly flowed towards the craterlet. When the dome was checked again on 1 April lava had filled most of the inner crater (nearly 60 m deep) and increased its area to nearly 600 m2. Assuming that the lava started to flow towards the craterlet on 25 March, and that it had been almost filled by 1 April, a rough estimate of the lava extrusion rate is 5,000-6,000 m3/day.

The formation of this craterlet was described in detail by Dr. Atl, the painter-volcanologist who later studied the Parícutin eruption in detail. According to him, the bottom of the volcano crater was almost flat before 1919. That year, extruded lava formed a small dome ~35 m high and 60-70 m diameter in the base. That dome collapsed in 1923 forming the craterlet. The volume of the internal cone of the craterlet is estimated to be 40,000 m3.

A series of SO2 flux measurements was begun after January 1994 (BGVN 19:11 and 19:12). During 1995 measurements rose to nearly 8,000 metric tons/day (t/d) in March, but gradually decreased to 2,000 t/d in June. A persistent decrease in gas emissions starting in July reduced the SO2 flux to nearly 100 t/d by December 1995. During the 5 March 1996 event, renewed ash emissions coincided with SO2 fluxes of up to 15,000 t/d; by late March it was decreasing, but emission levels remained high (>5,000 t/d). Currently, the COSPEC measurements are carried out by the Instituto de Geofisica (National University of Mexico), sponsored by the Secretaria de Gobernacion (Ministry of the Interior) through CENAPRED (Disaster Prevention National Center) using an instrument borrowed from the University of Colima and a plane owned by the Mexican Navy.

On 29 March the growth of a viscous lava dome was observed during a COSPEC flight (BGVN 21:03). The dome grew rapidly afterwards, and ash emissions from a NE-SW fracture along the SE inner wall of the main crater continued intermittently. Apparently, the emission center of the new dome is located between this fracture and the center of the small inner crater formed during eruptions in the 1920's.

During helicopter overflights on 10, 12, 24, and 29 April, gas emissions did not allow a clear view. The height of the dome was difficult to estimate, but was at least 50 m. The dome was also growing horizontally towards the NW with a steep terminal flow front. On the SE it was leaning in part directly against the inner wall of the main crater. The small old inner crater had been totally covered by the new dome. By comparing pictures of the dome formed in the 1920's with the present dome it is clear that the present dome is already much larger.

On 30 April at 1319 a major explosion from the dome dispersed ejecta to the NE. Maximum clast diameter was 0.5 cm in the village of Xalitzintla, ~12 km NE, and sand-sized ash fell in Tlaxcala, 60 km away. Because of bad weather conditions the explosion and accompanying phenomena were not recorded by the surveillance camera. Apparently, the ejecta were warm when falling in Xalitzintla. The shower on Xalitzintla lasted for ~2 minutes. Preliminary inspection of the material indicates that it was mostly light gray juvenile dacite, very glassy with incipient vesiculation.

Five climbers who ascended the volcano in the early morning hours of 30 April were killed by the explosion later that day. On 2 May the climbers were found a few hundred meters below the NE crater rim. Their corpses, recovered by Civil Protection authorities, exhibited 3rd-degree burns and severe injuries caused by contusions. Climbing the volcano has been officially prohibited since the current eruption began, and signs were posted at Paso de Cortes.

During a helicopter flight on 3 May a depression was observed on the surface of the new dome near the SE inner wall of the main crater. Streaks of gravel and boulders running down the NE outer slopes of the volcano were 10-20-m wide and a few hundred meters long, and very close to the route of ascent taken by most climbers.

Satellite observations. Thin steam/ash plumes were observed on visible satellite imagery and by surface observers at the Puebla airport during the first half of April. Plume heights were estimated to be from just above the summit (~5.5 km elevation) up to 7.5 km altitude. Prevailing winds generally blew the plume NE or E; it often remained visible on imagery for 25-50 km before dispersing. A larger plume on 11 April extended ~80 km E at 7.6 km altitude. A thin ash plume on 13 April was visible 130 km ENE. Except for one ground report late on 18 April, there were no satellite or ground observations of ash plumes during 16-26 April. However, aviation notices of the volcanic hazard remained in effect. Volcanic ash moving E and SE at summit level was again seen from the airport beginning on 27 April; cloud cover prevented satellite observations. The plume from the 30 April explosion remained visible, although it was thinning, into that evening as it drifted over the Gulf of Mexico. Aviation notices from Mexico City and Miami, Florida, warned of possible ash up to 12 km altitude. Ground and satellite observations of ash plumes continued into May.

Seismicity was monitored using the N-flank PPM seismic station at 3,900 m elevation. Seismic events were classified into three types (A, B, and AB) according to the definitions given in BGVN 19:01. Type-B (low-frequency) events dominated; between March and June a total of 11,678 type-B events was recorded, but only 17 type-A and seven type-AB events were registered. Numbers of daily type-B events varied greatly during March-June (figure 15). The highest and lowest values occurred on 18 May (209 events) and on 2 June (31 events), respectively.

A new eruption began on 5 March and an explosion on 30 April killed five climbers (BGVN 21:03 and 21:04). A new lava dome was first seen on 29 March and was estimated to occupy ~1.1 x 107 m3 on 26 May.

At 0905 on 28 October, the largest ash emission since 5 March (BGVN 21:03) was detected by a surveillance video camera. CENAPRED seismic and tilt monitoring network recorded the event as an emergent signal that produced high-amplitude records on the stations closest to the crater until 0913, when both amplitude and ash emission rate decreased. Heightened levels persisted until 0953, when the emission stopped, but high-frequency tremors continued for the most of the day.

The ash plume rose 2-3.5 km above the summit and was rapidly bent towards the W by the 30-45 km/hour winds. At 1630, NOAA/NESDIS reported that the ash cloud was between 5.3 and 6.3 km altitude. Minor ash falls were reported over towns W and NW of the volcano as far as 50 km away, and significant amounts of ash were observed on the volcano flanks and over the summit glacier, darkening the snow. Due to clear weather, visibility was >30 km and residents in towns on the W flank of the volcano witnessed the event; the explosion was also documented by Mexico City TV.

On the morning of 29 October the gas plume was drifting W at an altitude of 4.5-5.8 km. That day Jose Luis Macias, Claus Siebe, and Hugo Delgado of the Instituto de Geofisica of the University of Mexico (UNAM), Alejandro Mirano and Enrique Guevara of the National Center for Disaster Prevention (CENAPRED), and Mindy Brugman of the Columbia Mountains Institute of Ecology (CMI) observed the volcano using a helicopter provided by the Procuraduria General de la Republica. The helicopter flew at 6-6.5 km altitude to view the interior of the summit crater. The crater was still occupied by the lava body that first appeared in March 1996 (BGVN 21:04) and stopped growing by July. The concave lava body with a depressed central part observed during a reconnaissance flight on 25 October showed no substantial changes, but a new vent with an elongated trend NW-SE appeared on the central part of the lava dome. The dimensions of this craterlet were ~50 x 20 m (25% of the dome dimension) and 10-20 m depth.

Intense fumarolic activity was also detected within the SE part of the crater. Several other fumarolic sources were observed along the edge of the lava dome, at the crater wall and slope contact with the S side of the crater, and at the cracks on the lava dome. Along a N-S trending lineament a small amount of gases came from the SE flank at an approximate elevation of 5,000 m.

Although difficult to establish as a causal relationship, three A-type earthquakes at 0258 (M 2.2), 1728 (M 2.5) and 1811 (M 2.6) on 26 October may now be interpreted as precursors of this event. Similar A-type activity in the past was not necessarily followed by ash emissions. No other clear precursors were observed by the CENAPRED monitoring network. However, the explosive event of 28 October showed precursory evidence in terms of the SO2 flux and deformation of the volcanic edifice. The SO2 flux had averaged 11,000 metric tons/day (t/d) for the last 4.5 months, with a pattern of increasing flux starting on 30 September.

A peak in SO2 flux occurred on 21 October with >27,000 t/d recorded. By 25 October the SO2 flux decreased to 9,000 t/d, a 60% drop within three days. Two GPS stations located on the W flank of the volcano also started to show a continuous increase in the vertical (z) component since 5 October; this surface deformation pattern of uplift roughly followed parallel to the SO2 flux, having a peak on 21 October when the volcano suddenly rose at a rate of 40 mm/day. The vertical motion decayed in the same way as the SO2 flux pattern within the three days preceding the explosion. The correlation between the SO2 flux data and deformation data (derived from GPS) suggested repeated entrance of new magma into a large magma chamber beneath the volcano. The SO2 flux measurements are currently carried out under the sponsorship and collaboration of the Ministry of the Interior (CENAPRED) and Instituto de Geofisica (UNAM). Permanent GPS monitoring is carried out as a joint project UNAM-University of Miami/RSMA-MGG with funds from the NASA-DOSE program and support from CENAPRED.

Another ash-emission event occurred on 29 October at 2211. From the RSAM recordings, it was estimated that this new event released about one-fifth of the seismic energy of the previous one. The highest intensity phase in this case lasted ~7 minutes and then declined. Under clear moonlight conditions, local observers reported a similar short-lived plume rising ~2-3 km above the summit. Winds blew the ashcloud W, but no ashfall reports were received.

Popocatépetl at this stage efficiently releases the gases in a passive way and accommodates deformation due to intrusion of new magma. On 5 March 1996 the volcano started a new episode of activity with ash emissions comparable to those of December 1994 (BGVN 19:12). By the end of March, a lava dome was growing at the bottom of the crater. The dome continued growing at the moderate rate of a few cubic meters per second, as determined by aerial photogrammetry done by B. Cabrera from SCT, the Ministry of Communications and Transport, until July 1996, when the growth rate slowly declined. By September, the rate of growth could not be measured and ash emissions became smaller and less frequent.

Popocatépetl erupted on 25 December 1996 beginning around 1145 as seen in satellite imagery and discussed in pilot reports from American Airlines. Comparison of satellite imagery at 1145 and 1215 suggested that the eruption was not continuous.

About 1145 on 25 December the ash cloud was bounded by the following points: 19.0°N, 98.6°W; 19.2°N, 98.8°W; 19.3°N, 98.6°W; and 19.1°N, 98.4°W. At around that time GOES-8 data indicated that the eruption's ash plume moved N and E while dispersing rather quickly in both infrared and visible imagery. At 1645 that day the ash plume appeared nearly linear. It formed a band ~50 km wide trending WNW-ESE (from 20.2°N, 99.2°W to 19.6°N, 97.1°W) and covering a distance of ~230 km.

The plume from the prior day's eruption was indistinguishable by 26 December, based on GOES-8 infrared imagery. A SIGMET valid during 2300-2400 on 25 December indicated only near-source ash, suggesting that by this time conditions had return to normal. No additional eruptions were seen around that time.

A series of non-technical reports covering the volcano's behavior during the interval 17 February to 4 April are summarized in table 4. During this interval the hazard alert remained at yellow on a scale that encompasses the categories green (low), yellow, and red (high). The summaries document a pattern of isolated exhalations (some bearing ash) and occasional type-A seismic events. Noteworthy exhalations occurred on many days in the last three weeks of March. For example, on 20 March a relatively large exhalation lasted 7 minutes and spawned an ash column that rose to 4 km above the vent.

During 15-16 February activity was low and stable. Seismicity was also low but there were indications of isolated exhalations.

19 Feb 1997

Activity remained similar to that described in on the 17 February report; a type-A event of very small magnitude occurred at 0727.

25 Feb 1997

During the interval 1017-1130, tremor occurred accompanied by two intense exhalations (at 1029 and 1031). The exhalations produced a column of gas and ash that reached up to 3 km in altitude drifting to the SE. At 1239 a less-intense exhalation occurred but gas and ash were still emitted over a prolonged interval.

26 Feb 1997

After a short period of relative tranquility, during the previous night there was a moderate increase in the level of exhalations. At 0620 on 26 February a major exhalation occurred followed by prolonged tremor. A second large exhalation at 0915 accompanied increased tremor. Starting on 25 February, the volcano produced a plume of variable intensity that occasionally contained ash.

27 Feb 1997

Overall activity fell considerably compared to yesterday. Seismicity was limited to some weak exhalations and three type-A events of very low magnitude. Plume diminished in size and density.

28 Feb 1997

Stable activity such that in the last 24 hours there were only occasional exhalations and two type-A events of very low magnitude.

10 Mar 1997

During 8-9 March, activity remained stable without important changes. A few type-A events with low level magnitudes were registered. The plume was very small.

12 Mar 1997

At 0430 a moderate ash-bearing exhalation occurred associated with light tremor, which continued until the time of the report. High-velocity winds led to light ash rains on towns near the volcano. Local authorities were informed. Up until the time of the report, activity remained moderate without the threat of danger.

14 Mar 1997

General activity has decreased compared to that of 12 March. There persisted a white plume of small volume and on average, six small exhalations per day. Seismicity remained moderate except for type-A events: one, this day at 1130 (M 3.3) and the other, the previous day at 2130 (M 2.3).

17 Mar 1997

During 15-16 March there was a significant increase in exhalations both in terms of number and size, with an average of 50 per day. Between exhalations, type-A events occurred, many of low magnitude (only three reached near M 3). The larger earthquakes took place on 14, 15, and 17 March. The volcano produced a grayish-white plume. This level of activity was similar to that of April and May of 1996.

18 Mar 1997

The general level of activity stabilized, tending towards low values with respect to those seen yesterday. Although in minor proportion, the exhalations and type-A events persisted. The plume lacked significant changes.

19 Mar 1997

During the night, and today, the number and size of exhalations increased and at 0750 tremor occurred. It accompanied a plume that probably carried a moderate amount of ash.

The Satellite Analysis Branch of NOAA (SAB) conveyed several messages about ash plumes during February-March 1997. In one case, aviators near México City on American Airlines flight 1211 reported ash at 1500 on 5 February reaching 9.1 km. At 1715, GOES-8 satellite imagery indicated a detached plume ~90 km to the volcano's E; the plume was 45 km wide and at 7.6-9.1 km altitude. After that, Air Traffic Control in México City received no further pilot reports, and according to SAB, the plume dissipated in GOES-8 imagery around 2015.

A message from a United Airlines flight on 8 February around 0945 noted ash above México City at ~9.1 km altitude. Another pilot report noted that at 1715 there were narrow bands of ash at unspecified distances from the volcano at 5.5-6.1 km altitude, and farther S between 7.3 and 7.9 km altitude. A SIGMET issued in México City (valid during 1210-1800) warned of ash 28 km NE of the volcano between 5.2 and 6.4 km altitude. In addition, at 0945 a GOES-8 satellite image showed a plume near the summit extending S. By 1600 the eruption had stopped and radiosonde data established the plume at an altitude of ~9.4 km. At that time, the plume had become barely visible and rapidly fading on infrared imagery; it had moved 37 km SSE.

The third message, from a United Airlines flight at 0815 on 12 March, noted an eruption then. GOES-8 satellite imagery indicated a plume oriented ESE (on a bearing of 110 degrees). The plume extended for 70 km by 0945; at that time it was very narrow, linear, and at ~7.9 km altitude. Later, at 1115, a faint fan-shaped plume reached 55 km wide at 135 km SE of the volcano. The plume became covered by high-altitude weather clouds by 1315.

In addition, one available CENAPRED weekly seismic report (Number 59) covered the interval 7-13 April 1997; it showed a) the seismic and tiltmeter network (figure 16), b) computed epicenters during that time (figure 17), and c) cumulative seismicity for the interval 5 February 1996-15 April 1997 (figure 18). Figure 17 illustrates that many of the epicenters plot ~10 km SE of the crater. The epicenters SE of the crater caused investigators to ask if these earthquakes arose from volcanic or fault-related sources. To address this question, the investigators planned to deploy a network of portable broad-band seismometers to better cover this area.

Figure 18. Number of weekly Popocatépetl earthquakes counted by visual inspection and using Xdetect software, 5 February-15 April 1997. The number of times alarms were triggered is also shown. Courtesy of Roberto Quaas, CENAPRED-UNAM.

Figure 18 shows monthly totals both for earthquakes counted by eye directly from the seismograms (higher bars) and by using software called Xdetect (lower bars). Although this latter technique detects events automatically, the totals (which depend on the trigger thresholds) are much smaller than counts made by eye.

A series of non-technical reports covering the volcano's behavior during the interval 20 March to 30 April are summarized in table 5. During this interval the hazard alert remained at yellow on a scale that encompasses the categories green (low), yellow, and red (high). The summaries documented a pattern of isolated explosions and occasional type-A seismic events.

Microseismicity increased slightly on 23 April and at 1226 on 24 April a 15- minute explosive eruption sent an ash plume 4 km above the summit. Another smaller emission followed within 30 minutes. The wind carried the plume E. Activity decreased again on 25 April.

28 Apr 1997

Sporadic emissions of variable intensity. The largest on 26 April blew E and deposited ash.

29 Apr 1997

Major explosive eruption at 0110, followed by minor ones at 0122, 0159, and 0407. The first event caused ashfalls E of the volcano. Incandescent material was observed in the proximity of the volcano.

Three large explosions on 20 March and 24 and 29 April sent ash up to 4 km above the summit and caused tephra-falls on the NE slopes of the volcano.

The Satellite Analysis Branch of NOAA (SAB) conveyed several messages about the ash plume of April 24. The plume was seen in visible imagery moving E; one hour after the eruption started the plume was 65 km long and 8 km wide at an estimated height below 13 km. The plume remained visible in multi-spectral (MS) imagery for most of the following day, when a wedge-shaped ash cloud (maximum width at the leading edge reached 148 km) was moving NE at 100 km/hour at an altitude of ~6-8 km.

During the following weeks press agencies reported a major increase in activity. A sensationalistic Reuters news report described as "one of its most intense eruptions" of the past three years took place on the late evening of 11 May, with ash fall as far away as Veracruz city, 280 km E of the volcano.

On 27 May a message from a United Airlines flight noted ash moving ENE at ~13.3 km altitude, presumably related to another explosion. Visible satellite imagery the same day confirmed the presence of ash reaching the Gulf of Mexico and convective debris N and W of the summit.

The following includes summaries of reports from a) the Institute of Geophysics at the University of México (UNAM), b) the Centro Nacional de Prevencion de Disastres (CENAPRED), c) the NOAA Satellite Analysis Branch (SAB), and d) the United Nations Department of Human Affairs (DHA). This report covers the period from 2 May to 25 August. The most forceful emission in the 1994-97 episode took place on 30 June; ashfall shut down the Mexico City airport stranding passengers and spurring numerous press reports.

A series of non-technical reports during 2 May to 25 June (table 6) described isolated explosions and occasional A-type seismic events in a pattern that has characterized Popocatépetl's behavior since September 1996. A cross section shows the location of the volcano-tectonic earthquakes that occurred during 29 April-29 July; a table lists their locations during August.

The level of activity remained low, with sporadic low-intensity emissions and white plume.

05 May 1997

At 0839 there was a moderate emission of ash that generated a column ~2 km high drifting to the W. Ashfall was reported in the towns of Tepetlixa and Ozumba.

07 May 1997

On 6 May a major ash emission occurred at 2039 and lasted 20 minutes. The cloud drifted toward E and NE causing ash and coarser tephra to fall in Cholula and some areas in Puebla and Veracruz.

14 May 1997

On 13 May at 2230 a moderate emission included incandescent fragments that fell near the crater. Ashfall started afterwards on the towns of San Pedro Benito Juarez, San Nicholas de los Ranchos, Calpan, and Santiago Xlizintla, where weak earthquakes were also felt.

24 May 1997

After several days of relative quiet a high frequency tremor was recorded at 0927. In the meantime ash was emitted up to 200 m above the crater. The plume drifted to the ENE causing minor ashfall in the towns of Calpan, Xalitzintla, San Nicolas de los Ranchos and Nealtican.

11 Jun 1997

At 1014 a 15-minute-long tremor accompanied a major ash emission that reached an altitude of 4 km (see figure 19). The column blew towards the WSW.

18 Jun 1997

Activity was again at low levels. When inspected by helicopter, the summit glacier appeared normal.

25 Jun 1997

The activity was at stable, low levels, with minor emissions and an almost constant presence of a low steam plume on the summit.

Activity during 2 May to 25 August 1997. Large ash emissions occurred on 11, 14, 15, 24, and 27 May and noteworthy or large emissions occurred on 3, 11, 14, 19, 21, and 30 June. On 28 May satellite imagery showed an ash cloud moving rapidly SE as it approached the Yucatan peninsula.

On 11 June ash streamed S of the volcano at 28 km/h. The cloud measured 50 km long and 33 km across (figure 19). The following day ash was reported at an altitude of 6-8 km; thicker ash closer to the volcano moved S at ~50 km/hour while an area of very diffuse ash headed SW. The 14 June eruption was visible from both Mexico City and Puebla; satellite imagery showed the plume heading WSW at ~40 km/hour. The plume later separated: a thicker L-shaped area fanned NW to W at 30 km/h at an altitude of ~10 km, and a faint area of thinning ash moved W at ~64 km/hour ahead of a thick-ash area at 7-km altitude. Reports of sand-sized ashfall came from Nepantla, Amecameca, and other towns as far as Cuautla. On the Puebla side of the volcano several towns reported mudflows associated with heavy rains and minor melting.

Figure 19. Popocatépetl ash column; photo taken from the NW (above Paso de Cortes) at 1032 on 11 June 1997. See table 6 for a brief description of the ash emission. Courtesy of CENAPRED.

On 12 June Tom Casadevall noted that he had learned from an engine manufacturer that ". . . all three major Mexican airlines (Mexicana, Aeromexico, and TAESA) have reported windshield damage that they attribute to volcanic ash. Also, Aeromexico reported heavier than normal blade erosion on one MD80 engine that it attributes to ingestion of volcanic ash from Popocatépetl. Apparently the local atmosphere now contains an above average concentration of ash."

The 30 June ash emission was the largest recorded since the current eruptive episode initiated in 1994. Beginning at 1656 on 30 June there were seven volcano-tectonic earthquakes (M 2-2.7) in a 13-minute interval. At 1711 a large tremor signal marked the eruption's start. The first pulse lasted 135 minutes. The second one, beginning at 1926, lasted about 90 minutes. The latter eruption sent an ash plume 13 km above sea level within minutes. About 2-3 hours later, ash started falling over many towns around the volcano, including Mexico City.

In spite of the outbursts during this eruptive episode, estimated to a VEI of 2-3, no casualties or damage were reported; the volcanic alert code was raised to red but no evacuation was involved. The airport in Mexico City was closed for about 12 hours, until the ash was washed away from the runways. Pumice fragments as large as 10 cm fell sparsely on the N flank at Paso de Cortes and over a few kilometers along the road to Amecameca.

According to the real-time seismic amplitude measurement recordings (RSAM), the 30 June event alone released an estimated energy equivalent to one-tenth of the seismic energy release during an average year. The highest intensity phase lasted about 35 minutes and then declined.

During the two days following the eruption, some minor mudflows were reported at the town of Santiago Xalitzintla, about 12 km NE of the volcano. The flows coincided with heavy rain inundating a small area in the bottom of a ravine where a small house partially flooded. Inspection of the house, local fruit trees, and a small corn field in the area, showed that the flow was rather slow. After the major ash emission on 30 June, the volcano quieted. Steam emissions continued, at times accompanied by ash; these emissions were small except for a relatively large event on 2 July.

Helicopter observations on 3 and 4 July disclosed new features. There were several 1- to 2-km-long tongues radiating down the volcano's S and SE flanks. These tongues were interpreted as granular flows produced by partial collapse of the eruptive column. Inside the main crater on the 1996 lava dome there was a new crater enclosing a fresh ropy-lava body. As a preliminary interpretation, it seemed that in the first stages of the 30 June event the previous dome was partially destroyed by explosions, forming the initial crater. Then the crater was flooded with fresh magma that apparently underwent significant fragmentation, generating the moderately large ash emission and leaving a new lava body with a conical depression. In response to these events, a UNDP/DHA Resident Representative reported on 4 July that preparedness measures were undertaken. CENAPRED provided ongoing information to the villages on the outskirts of the volcano (total population, 102,000).

After 30 July, activity decreased until 12 August, when a moderately large emission discharged ash 5 km above the crater. By another account the ash only rose 2 km. This emission lasted for more than two hours and produced SW-flank ashfall. After this event the color of the volcanic alert light remained yellow. During the afternoon another 3-minute emission sent an ash plume to 2.5 km above the summit.

Activity remained low until 25 August but included frequent low- to moderate-intensity gas-and-steam emissions, some with small amounts of ash. Around this time, the highest number of emissions per day was 41 on 21 August.

Low-frequency tremors of variable duration (between 2 and 40 minutes) occurred sporadically during this period. Figure 20 shows the hypocenters of the volcano-tectonic earthquakes located during March-July; table 7 lists those during August.

Figure 20. Cross section of Popocatépetl made from a perspective of looking towards the N; it shows the hypocenters of the volcano-tectonic earthquakes located during March-July 1997. The numbers key to the day of occurrence (see box), the dot sizes are proportional to the magnitude (no scale given). Vertical exaggeration is 2:1. Courtesy of CENAPRED.

Table 7. Occurrence of local volcano-tectonic earthquakes at Popocatépetl during August 1997. Courtesy of CENAPRED.

Date Magnitude Location 13 August 2.3 4.4 km under SE flank 14 August 2.2 6.8 km under the summit 14 August 2.5 5.3 km under the summit to the SE 17 August 2.4 SE region 19 August 2.1 7.3 km under the summit to the NE 19 August 1.7 4.6 km under the summit to the E. 20 August 2.6 5.8 km under the summit 20 August 2.2 5 km under the summit 20 August 2.3 5.7 km under the summit

Popocatépetl continued to generate small and moderate emissions and low-level seismic activity following the large ash emission of 30 June (BGVN 22:07). The volcanic alert status remained yellow through August, September, and October. The following, taken from CENAPRED daily summaries, outlines the characteristic emissions and seismic activity from mid-August to the end of October. Also during this period, a small lava dome grew in the crater, and scientists installed additional monitoring equipment.

Moderate to large emissions were frequently observed during 12 August-31 October (figure 21). Characteristic low-level venting of gas and steam were an ongoing background activity. The large ash column of 12 August, considered the most significant since 30 June, was succeeded by a week of frequent, small emissions of gas and steam occasionally bearing ash, although poor visibility impaired observation of the summit. On the 18th a column of ash rose 500 m above the summit. Fumarolic plumes persisted during 20-24 August and on the 26th an ash column rose to 1 km. Frequent moderate emissions of gas and steam prevailed from 27 August to 2 September. The climax occurred on 30 August coincident with seven harmonic tremors in an 8-hour period. Observed emissions decreased thereafter, generally remaining stable at diminished levels until the end of September. A persistent gas-and-steam plume was the only activity observed during 20-24 September; observation was hindered by bad weather on the 25th. Starting on 28 September activity began to increase, but again observation was limited by bad weather. In early October small amounts of light ash may have been produced.

Beginning on 4 October moderate emissions included light ash; these increased steadily for a week. Steam plumes from a crater fumarole were seen on 8 and 10 October, but no important changes to the summit or glacier were observed. Activity was variable on the 9th as a hurricane passed by on the Pacific coast, obscuring the mountain, and on the 12th a plume rising to 1 km was dispersed NE. Activity declined steadily after the 12th to the end of October, except for a few isolated increases. Gas emissions can be gauged by the measurement on 20 October of 9,000 metric tons/day of SO2. On both the 22nd and 27th short increases in the number of emissions were followed by harmonic tremor. Ash plumes were seen on the 24th and 26th.

Almost daily tremors lasted a few seconds to many minutes. Several A-type earthquakes were distributed through the period (figure 22). The thirty-five events recorded ranged from M 1.75 to 2.70 at depths of 3-8 km, mostly under the summit or in the direction of the SE flank. Several A-type earthquakes, other disturbances, or both occurred on 19 and 20 August, 8, 10, 26, and 28 September, and 24-26 October. On 20 October four A-type events were followed by 15 minutes of harmonic tremor.

Aerial photos of the crater had revealed a small growing lava dome possibly related to harmonic tremor episodes on 17 and 18 August. During a helicopter flight on the morning of 3 September, scientists noted an increase in the lava dome, although it still filled <20% of the crater. Lava was keeping the vents closed so there was little fumarolic activity. Three minutes of large-magnitude harmonic tremor registered on 11 September likely indicated magma movement or growth of the lava dome. No change was seen during a helicopter flight on 12 September, but on the 18th scientists observed radial growth in the dome. Observers noted that the dome was keeping vents partially obstructed.

Slight tilt changes recorded by inclinometers on 14 October possibly indicated a small deflation of the E flank. The next day tilt remained stable or in some cases returned to previous levels, but shifted again on the 16th and continued until the 21st. Tilts again shifted on 24 and 30 October.

As a joint project involving experts from CENAPRED and the Cascades Volcano Observatory, an automatic flow detection system (see table 8) was installed on the N flank of the volcano beginning 9 October. Due to bad weather and high seismicity only the repeater station, which will receive local signals and transmit them to CENAPRED, could be installed. Work continued on 14-15 October when the PFM3 and PFM1 stations were installed; PFM2 was installed on 7 November. Each station consists of a seismic flow detector, a rain gauge, conditioning circuits, and a data transmitter.

Low activity through November; lava extrusion and explosion in December

Low levels of eruptive and seismic activity characterized Popocatépetl through most of November. Typically, a few events occurred each day, including short episodes of low-amplitude harmonic tremor and gas- and-steam venting in plumes that drifted to the NE or SE. Tiltmeters showed little variation in November but indicated a slight increasing trend. Bad weather and poor visibility occurred frequently.

Table 9 lists type-A seismic events recorded during November. Two episodes of harmonic tremor were recorded on 1 November. A 2 November lightning strike disabled video monitoring until the 5th. Poor weather impeded observation on 11-12 November during a slight increase in activity. On 15 November, a slight increase in the number of events was accompanied by minor ash emission. Some ash was also emitted in conjunction with seismic events on 21 November.

Table 9. Type-A seismic events recorded at Popocatépetl in November 1997. Courtesy of CENAPRED.

On 24 November, seismic and associated eruptive activity began to increase. Thirty-six low- to moderate- intensity seismic events were recorded, including significant exhalations at 0823, 0829, 0857, and 0953; during these events, ash plumes rose to 1 km and drifted NE. Low-amplitude harmonic tremor 3-5 minutes in duration occurred in the afternoon. On 25 November, 42 seismic events were recorded; some were accompanied by ash emissions and periods of tremor lasting 2-8 minutes. No significant deformation was observed. During a 25 November helicopter flight, increased gas and steam from fumaroles obstructed views of the crater and dome. By 27 November, activity was subsiding; 29 seismic events and tremor 2-3 minutes long were recorded. Levels of seismic activity continued to decline until the end of the month.

The last of several flow-detection monitoring stations (BGVN 22:10) was installed on 7 November; also, a temporary high-gain broad-band seismograph was installed at the Canario station to study the N flank in more detail. The Canario station's instrumentation included a triaxial short-period seismograph, a triaxial broad-band seismograph, a digital inclinometer, a flow detector, and a rain gage. To reinforce the seismic and geodetic monitoring system, a new station was installed on 28 November on the W flank just under Ventorrillo peak near the Nexpayantla ravine at 4,452 m elevation. The instrumentation includes a triaxial short- period seismograph and a biaxial tiltmeter.

Two important eruption episodes highlighted activity in December. Both eruptions involved extrusion of lava into the crater, creating a dome that sealed fumaroles.

Small- and moderate-intensity emissions of gas and steam characterized activity at the volcano for most of December. Small type-A tectonic events occurred regularly along with incidents of tremor. The first increase in activity began 5 December with 15 small gas-and-steam emissions, tremor of 90 minutes duration beginning at 1335, and two type-A seismic events in the late afternoon. At 0315 on 6 December activity increased considerably and continued throughout the day. A moderately large high-frequency tremor accompanied by continuous gas, steam, and ash exhalations lasted until 1700. Early in the morning of the 6th a faint glow at the fumarole was observed through the video monitor, indicating the presence of incandescent material in the crater interior. In the late afternoon the gas emission abruptly stopped, possibly due to obstructions of the vents. Observers speculated that these phenomena indicated the extrusion of new lava in the bottom of the crater. No significant changes of the other measured parameters could be observed.

The following day saw a considerable decrease in activity: only six moderate emissions including some short ash puffs. Continuing very low emissions over several days indicated the vents were partially closed. On the morning of 9 December personnel from CENAPRED and the Instituto de Geof¡sica, UNAM, made a helicopter overflight during which the presence of a large lava dome, spread across almost over the entire crater floor, was seen. This observation confirmed the assumption of a new lava extrusion on 5-6 December. No important changes on the volcano flanks or the glacier could be observed. SO2 measurements made the afternoon of 9 December gave preliminary values of 6,100 tons/day. The other parameters that are continuously monitored showed little variation. Popocatépetl returned to characteristic low levels of activity, although some earthquakes of M 2.2 occurred at a depth of ~4.7 km during 14-15 December. On 13 December strong winds and low temperatures caused damage to monitoring equipment, including the video transmission link.

After several weeks of very low activity an eruption clearly observed from neighboring towns started at 1930 on 24 December. The activity began with a 2-minute explosion followed by 15 smaller volcano-tectonic events and several moderate emissions. According to reports from the nearby towns of San Nicolas de los Ranchos and Amecameca, during the first event observed brightness around the summit was produced by the expulsion of incandescent materials, and an associated shock wave was felt. From Puebla grass fires were reported on the E flank of the volcano. Ashfalls were reported starting at 2045 in towns E of the volcano (Atlixco, Calpan, and San Nicolas de los Ranchos). The whole episode lasted a total of 30 minutes. All monitored parameters except for seismicity then returned to normal levels. This eruption was probably associated with the reopening of conduits inside the crater, obstructed since 6 December by lava extrusion. This obstruction was carefully monitored because pressurization of the system raised the possibility of explosive events. Following the eruption of 24 December activity returned to very low, stable levels for the remainder of the month. The volcanic- alert system remained on "yellow" (caution) through all of December.

Following the 24 December 1997 eruption (BGVN 22:11) Popocatépetl had several days of low activity followed by two moderate type-A seismic events and an explosion on 1 January. The earthquakes occurred 5.8 and 7.5 km SE of the crater at depths of 4.4 and 4.7 km, respectively. An eruption at 1827 on 1 January (figure 23) sent a large ash column 5 km above the summit and caused ashfall on towns to the SE such as San Baltasar Atlimeyaya, Metepec, and Atlixco. Incandescent material was ejected NW from the crater and started grass fires (figure 23). The event lasted for 1.5 minutes; after 20 minutes, activity returned to normal. Later observations revealed that no substantial changes had taken place but that a flow-detection station on the Espinera ravine had been destroyed by rockfall.

Figure 23. Image of the N flank of Popocatépetl taken at 1832 on 1 January 1998. The bright area in the right-hand corner corresponds to grass fires. Courtesy of CENAPRED.

During 2-20 January, normal conditions prevailed; continuous gas-and-steam emissions, small ash exhalations, and occasional low- to moderate-intensity Type-A events were observed. On 6 January, a moderate exhalation at 0049 produced a glow and was followed by a sequence of moderate tectonic events leading to 8 minutes of small tremor. The seismic activity was probably associated with movement of the lava dome. A 3-minute tremor was recorded at 0105 on 16 January and on the next day a 2-minute sequence of volcanic events occurred. During 18-20 January, seismicity and the number of exhalations increased slightly.

In the weeks following a 1 January explosion that caused ashfalls and ignited grass fires (BGVN 22:12), stable activity prevailed. During 21 January-12 February, the volcano was the scene of low seismicity, low- to moderate-intensity exhalations, and moderate amounts of gas and steam.

On 26 January, a M 1.9 type-A tectonic earthquake occurred 1.5 km SE of the crater at a depth of 5 km. Two small 5- and 8-minute exhalations on 27 January sent ash puffs, gas, and steam ~1.5 km above the summit. Another eruption later that day dropped ash on a nearby village.

On 31 January, activity increased slightly when a 6-minute-long series of low-magnitude tectonovolcanic events began at 0104. These earthquakes were followed by several short-lived episodes of harmonic tremor. These events may have indicated movement of magma within the lava dome and magma extrusion; however, there was no significant change in activity afterwards. The volcano was not affected by a 3 February M 6.4 earthquake located on the coast of Oaxaca, Mexico. On 4 February, a M 2.2 tectono-volcanic earthquake occurred 10 km NE of the crater.

In the morning of 11 February, a larger exhalation that lasted ~2 minutes produced ash that was detected on radar images. After the event, activity returned to normal with small exhalations and light emissions of gas, steam and some ash. By 12 February, seismic and fumarolic activity showed a significant reduction; CENAPRED suggested that the reduction was caused by obstruction of the vents and that a new eruptive event was possible.

Popocatépetl, whose name is the Aztec word for "smoking mountain," towers above Mexico City and is the second highest volcano in North America. The stratovolcano is generally symmetrical, contains a steep-walled, 250-m-deep crater, and is modified by the sharp-peaked Ventorrillo on the NW, a remnant of an earlier volcano. The current summit was formed by repeated lava effusions until about 1,200 BP, after which the current, dominantly explosive phase began. Frequent historical eruptions have been recorded since the beginning of the Spanish era.

Cyclical dome extrusions that by late 1997 filled one-third of crater capacity

The following report on Popocatépetl incorporates both background descriptive information, some of which had previously remained unreported, and a more detailed discussion of ongoing dome growth based on aerial photographs and flight observations. The volcano was last discussed in BGVN 23:01. By late 1997 the growing dome occupied 30-38% of the crater's capacity.

During 1996-98, Popocatépetl extruded six named domes in the summit crater (A through F, table 10 and figure 24). Elliptical in shape, the summit crater measures 820 x 650 m, with the longer axis trending approximately E-W. The lowest point of the crater rim occurs along the NE side and lies at 5,180 m elevation; the average elevation of the irregular floor was estimated at 5,030 m (De la Cruz-Reyna et al., in review). The crater's deepest point, at 4,963 m elevation, lay at the bottom of the ~160-m-diameter craterlet formed during the 1922 eruption (BGVN 21:03). Based on the observed shapes and dimensions, the crater could potentially contain a volume of ~35 x 106 m3 before additional material would spill out the low point on the crater rim.

Table 10. Approximate dates when the first extruded material was seen for Popocatépetl's domes A through F. Courtesy of CENAPRED.

Dome Birth date Comment
A late March 1996 --
B 21 May 1996 --
C 21 January 1997 Higher viscosity lavas than domes
A or B.
D 04 July 1997 Followed the unusually large 30 June
1997 explosion that left a large
crater in dome C.
E 19 August 1997 --
F 07 December 1997 --

Figure 24. Schematic plan views showing the main crater at the summit of Popocatépetl and the sequence of named domes (A-F) found during 26 May 1996 through 7 December 1997. Courtesy of CENAPRED.

In late March 1996, observers saw dome A growing at the bottom of Popocatépetl crater and slowly covering the 1922 craterlet (BGVN 21:03). By 21 May 1996, two elliptical lava bodies were observed in the main crater of Popocat'petl, completely covering the older dome and craterlet (BGVN 21:04). As shown on figure 24, domes A and B grew along the SE and NW sectors of the principal crater's floor (BGVN 22:10). By 26 May 1996 the highest point on dome B reached 5,109 m elevation. Then, after July 1996 dome B's moderate growth slowly declined and subsequent circular fractures on the central dome indicated subsidence. By September 1996 the growth rate could not be measured and ash emissions became smaller. After September 1996, explosive emissions became less frequent, but more intense (e.g. those on 28 and 31 October 1996, BGVN 22:10).

By 21 November 1996, dome B had covered most of dome A and it crept radially out towards the crater's walls. Apparently, explosive activity around that time caused enhanced central subsidence as concentric fractures returned to the dome's surface and the elevation of its central part fell to 5,090 m. More explosions were recorded on 27, 28, and 29 November, on 2, 5, 7, and 29 December, and on 5, 12, 17, and 19 January, 1997. The January explosions were noted as large. By 21 January observers reported that dome B's previously irregular surface appeared smooth due to a cover of fresh tephra. More surprisingly, the central depression within dome B increased in depth, creating what looked like a new crater.

More explosions soon followed (on 23 and 29 January, and on 4, 5, 8, and 25 February; BGVN 22:03). Next, new lava extruded at the center of the depression constructing a new, smaller dome (C). The lavas comprising dome C appeared to have a greater viscosity than those of either A or B.

Explosions on 19 and 20 March 1997 (BGVN 22:04) failed to remove significant proportions of dome C; by 23 April dome C's central part reached 5,060 m elevation (figure 24). As previously reported (BGVN 22:04 and 22:07), subsequent explosions (24 and 29 April, 11, 14, 15, 24, and 27 May, and 3 and 11 June 1997) partially destroyed dome C leaving it covered by explosive clasts of very different sizes. Moreover, the central part of dome C had subsided, leaving its lowest point at 5,049 m elevation. More explosions on 14, 19, 21, and 30 June and on 2 July thwarted observations of the crater's interior. The 30 June 1997 explosion, the largest since the eruption began in 1994, quickly dispatched an ash column to 13 km altitude (BGVN 22:07). When observers looked into the crater on 4 July 1997, dome C had been partially destroyed and contained a large crater.

Within that crater there lay a dish-shaped zone of fresh ropy-lava given the name dome D. In addition, tongues of material radiated from the crater over the volcano's S and SE flanks; these were interpreted as granular flows deposited by the 30 June eruption (BGVN 22:07). Although not previously reported, on 10 August subsidence and radial fracturing became more evident on dome D. Later, by 19 August, dome D sprouted additional lava thus forming what was termed dome E (BGVN 22:10).

Dome E, initially an elliptical lobe that was 50-m long, 36-m wide, and 6-m high, had a very rough surface texture. Dome E later attained a circular shape, and by 10 September it had almost filled the hosting craterlet within the surrounding dome's body. Apart from some radial fractures, the surface appearance was rather regular with a slight inner depression and a region emitting gases in the center. This circular center had a height of 5,105 m elevation. From then on, E extruded in a piston-like manner and when seen on 22 October, E retained an almost cylindrical shape: Its height had grown about 15 m without significant change in its horizontal extent. When viewed on 29 November E's surface appeared smoother except for the presence of some minor explosion craterlets.

Starting on 25 November, significant seismic changes indicated subcrater magmatism and on 2 December observers noted both mild ash emissions and night-time incandescence. On 7 December observers recognized yet another new, large lava body in the crater (BGVN 22:11).

Dome F was composed of a lower-viscosity, black, ropy lava; it subsequently grew to a maximum diameter of 380 m and exceeded by 20 m the height of dome E as measured on 22 October. Relative quiet during 7-24 December ended on the latter day with a 30-minute-long series of explosions and moderate ash emissions. Volcano-tectonic seismicity took place during the final days of 1997, leading up to a large 1 January explosion. Aerial observers on 6 January saw that dome F had been partially destroyed and covered by volcanic debris (BGVN 22:12). The negative values on table 11 correspond to the 1 January 1998 explosion, which left a crater at dome F's center. This crater was 250 m in diameter and 60 m in depth with a shape similar to the 1922 dome and craterlet. Dense, degassed lava blocks with diameters of 0.6-0.8 m were thrown 2 km from the crater; they produced impact craters about 3 m in diameter.

Table 11. Estimates of Popocatépetl dome volumes for the stated dates. Volumes are "actual" and not adjusted as dense rock equivalents. The maximum crater capacity is estimated at ~ 35 x
106 m3. The negative emitted volume shown for 1 January 1998 appears because explosions removed material from the dome, although some uncertain amount of these broken dome fragments remained within the crater (see text).
Courtesy of CENAPRED.

Afterwards, until early February 1998, the volcano remained relatively quiet. On 14 March 1998, new precursory seismicity was detected. In behavior reminiscent of December 1997 and January 1998, two explosions occurred on 21 March at 0511 and 1559. The first, a moderately explosive exhalation, produced light ashfalls on towns in the state of Puebla. The second, a more intense explosion, produced a 3-km-tall plume and threw blocks 2-4 km about the crater. A 23 March exhalation appeared very similar to the one at 0511 on 21 March, resulting in a low-altitude plume that the wind dispersed NW. No damage or casualties were reported.

Activity at the start of April showed an increase over that of late March. Small emissions of gas, steam, and light ash along with rockfalls and short harmonic tremors were typical events at Popocatépetl throughout April and May. Smoke from forest fires, mist, and air pollution made direct observation of the volcano difficult for much of both months.

Although not shown as a located A-type event (table 12), on 1 April a gas-and-steam plume was observed rising 500 m above the crater and dispersing to the NE. At 1653 on 2 April seismometers and flow detectors sensed a disturbance that turned out to be a flow of water and debris traveling down NE-flank ravines. The flow was attributed to glacial runoff after days of intensely hot weather. The flow continued for hours and reached points 8 km downstream from the glacier. For 12 hours after this event seismic activity steadily decreased. Only small, short exhalations occurred until 0522 on 3 April when another A-type event occurred.

Activity remained low to moderate over the next week. At 0425 of 11 April there occurred a small harmonic tremor of 30-second duration. At 1632 the same day a medium-intensity exhalation was followed by 12 minutes of harmonic tremor. Other exhalations occurred that day at 1912 and 2321, and at 0235 the morning of 12 April, all of which were accompanied by small emissions of gas and steam.

At 1107 on 21 April a large exhalation took place that lasted 5 minutes and produced an ash column 4 km high. Although heavy clouds obstructed visibility, a video image of the ash column showed it clearly (figure 25). Incandescent fragments were ejected producing several grass fires on the upper slopes of the volcano. Vibration of windows in the city of Puebla was reported and some ash fell in both Cholula and La Paz. At 1453 the same day a similar exhalation occurred, but was smaller in magnitude and only 1 minute in duration. The last event possibly emitted ash, but this could not be confirmed because of limited visibility. Activity decreased slightly over the next few days, except for some A-type events. Bad weather engulfed the summit in cloud.

Figure 25. Picture taken at a Popocatépetl video monitor on 21 April showing emitted ash column. Courtesy of CENAPRED.

The broad-band seismometer recently installed at the Canario station (BGVN 22:10), located on the N flank at 4,300 m altitude 2 km from the crater, went out of operation 21 April probably because solar panels were damaged by ejected rocks. This is the second station damaged by volcanic activity this year (the first was the Espinera station, PFM2). Because of the danger of future explosions, an area within 4 km of the crater was deemed of high risk and restricted access.

On 24 April a large exhalation was recorded at 1257. It had a duration of three minutes and probably produced a small ash emission, which because of poor visibility could not be confirmed. At 1031 on 27 April another explosive exhalation occurred. The most intense phase lasted three minutes and was followed by high-frequency tremor that lasted an additional five minutes. It produced a 4-km-high ash column over the summit. Immediately afterwards the volcano returned to previous lower levels of activity. Mild ashfall was expected in the towns located to the E and NE of the volcano.

May began with a slight decrease in activity. Mist and clouds with occasional smoke persisted in obstructing visibility of the mountain. Activity increased slightly on 9 May and included seismic events at 0255 of 2.5 minutes duration, and at 0546 of two minutes duration, possibly due to small emissions of gas, steam, and ash. At 1205 an M 5.2 earthquake occurred on the coast of Guerrero and was recorded by all the stations monitoring the volcano. The event did not affect Popocatépetl.

Isolated low- to medium-intensity exhalations were recorded 10 May. The largest events occurred at 0744 (1.5 minutes duration) and at 0842 (one minute). Both events produced small emissions of gas and steam which rose to 1 km above the crater. Rockfalls were recorded on the N flank of the volcano. At 1322 a moderately large exhalation occurred, followed by four similar but smaller events during the next 5 minutes. Immediately after, one minute of low-frequency harmonic tremor of considerable amplitude was recorded followed by five minutes of high-frequency tremor.

Despite the limited visibility of the volcano at that time, an ash column with gas and steam could be observed rising several kilometers above the crater and was directed by low-speed winds to the NE. The appearance of the column was confirmed by observers viewing from a helicopter in the vicinity of the volcano. These events, due to their explosive nature, ejected solid and incandescent material from the crater over an area of 2-3 km radius. After the episode the volcano returned to previous levels of activity and remained stable for the rest of the day. No reports of damage or ashfall were received.

Beginning at 0426 of 11 May, a high-frequency tremor accompanied a gas-and-steam fumarole which ended with a moderate exhalation at 1505. The afternoon of 12 May at 1755 a column of gas and steam rose 2 km above the summit. The following day, emissions of gas and steam with slight amounts of fine ash generated puffs rising 500-1,000 m above the crater. At 1023 on 15 May three small exhalations mixed with harmonic tremors of low frequency occurred. Other tremor episodes lasting up to five minutes were recorded at 1030 and 1035. At 1043 a medium-intensity exhalation occurred which produced a puff of steam, gas and some light ash, dispersing to the NW. During the rest of the day several similar exhalations were recorded.

On 16 May at 1242 another strong M 5.2 earthquake located on the coast of Guerrero was recorded by all seismic stations at the volcano. This event did not produce any changes in the activity of Popocatépetl. At 1308 a small exhalation occurred accompanied by steam and followed by low-amplitude harmonic tremor lasting 6 minutes. Other short tremor episodes were recorded in the afternoon. During the next few weeks forest fires on the N flank of the volcano close to Tlamacas limited visibility.

The measured SO2 value on 30 March was 5,500 tons/day: on 27 April it was 10,600 tons/day. During the Holocene, Popocatépetl produced both effusive and pyroclastic activity. About 30 eruptions are known since 1345, although early documentation is poor. Most historical eruptions were apparently mild-to-moderate Vulcanian steam and ash emissions, with larger explosive eruptions in 1519 and possibly 1663. Activity in 1920-22 produced intermittent explosive eruptions and a small lava plug in the summit crater. Minor ash clouds were also reported in 1923-24, 1933, 1942-43, and 1947.

Stable with moderate steam, gas, and occasional ash plumes during June

Popocatépetl continued to be generally stable at moderate levels of activity through June. Small, short-lived emissions of light steam, gas, and occasional ash produced small plumes rising a few hundred meters above the summit on most days of the month. Mist and cloud sometimes obstructed visibility, but the pervasive smoke from fires in the region as reported in the last few months (BGVN 23:05) was no longer a problem.

Seismic activity was low to moderate. A-type events were recorded on 1, 6, 21, 23, 24, 26, and 29 June. They mainly ranged from M 2.1 to 2.5, except for one on 24 June, which was M 3.1. They were generally located ~5 km below the summit and ~7 km to its SE. Harmonic tremor episodes of 3-5 minutes were recorded on 21 June, and 4 minutes of tremor were recorded on 26 June. Starting at 0935 on 28 June seven minutes of weak low-frequency tremor was followed by 5 hours of similar but shorter events. Several episodes of low-frequency tremor lasting 1-5 minutes occurred the next morning. The large (M 5.7) earthquake, which occurred on the coast of Oaxaca and Chiapas on 7 June, had no detectible effect at Popocatépetl.

Sometime during the last week of May vandals raided Bonsai (PPB), a seismic station on the NE flank in the state of Puebla. They took recording, telemetry, and power-supply equipment. The loss of the station will affect the ability of Centro Nacional de Prevencion de Desastres (CENAPRED) scientists to locate volcano-tectonic events.

In April 1997, a "COSPEC Workshop" took place at Arizona State University. Participants, including scientists from Instituto de Geofísica, UNAM, brought instruments that were adjusted and tuned by Millan Millan (the inventor of COSPEC, a correlation spectrometer used to measure SO2 gas) and Bob Dick (an engineer for Barringer Ltd., the manufacturers of the COSPEC instrument). During tuning of the Instituto de Geofísica COSPEC instrument used to monitor SO2 emissions of Popocatepetl, a problem in the low-concentration calibration cell was found. This cell did not have the concentration given in the manufacturer's specifications, and in fact, its true concentration was far too low. Exchanging cells with another COSPEC and determination of the true concentration of the previously installed cell solved the problem.

The instrument with the incorrect calibration cell had been used in Mexico since June 1996. Thus, the flux measurements carried out between June 1996 and April 1997 were revised and recalculated using a newly determined value for the old low-calibration cell.

Most SO2 fluxes at Popocatepetl volcano are very high and thus, the low-concentration value is not used frequently. Thus, the database did not undergo significant modifications in terms of mean flux. However, some of the largest values were modified (became smaller) for cases where relatively low concentrations prevailed over extremely long distances.

The highest SO2 fluxes measured at Popocatépetl volcano are 50,000 tons/day, and the mean flux for the last four years is nearly 10,000 tons/day. The fluxes determined from 1994 to June 1996, and from April 1997 to present do not need revision because they were obtained with other COSPECs belonging to Arizona State University, Alaska Volcano Observatory, Universidad de Colima, and University of Montreal or because the calculations used the new low-calibration cell. The monitoring of SO2 fluxes at Popocatepetl volcano is routinely carried out 2-3 times a week under collaboration between (CENAPRED) and the Instituto de Geofisica (UNAM).

Preliminary measurements of SO2 values for June show an increase over May. No important ground deformation was recorded. The hazard status remained Yellow and authorities recommended staying beyond 4 km from the crater.

During July 1998 Popocatépetl issued sporadic minor-to-moderate exhalations, steam plumes, and occasional minor ash. The daily log for July posted on the Internet by CENAPRED (see below) shows that activity was very similar to that reported for June (BGVN 23:06). That report also discussed the discovery of a defective calibration cell in a spectrometer used to measure SO2 at the volcano. Fortunately, the required corrections needed only to be made for low SO2 flux conditions. Although some of these uncorrected low fluxes may have been cited in previous Bulletin reports, the volcano has typically produced high SO2 fluxes.

Summary of daily activity during July. These observations and hazards were posted on the CENAPRED web site. Some of the original text has been edited.

1 July: Activity remained stable with a tendency to decrease. The number of exhalations decreased slightly; they were of low-to-moderate intensity and sometimes accompanied by steam and gases. In the morning a small plume blew SW. The recommended minimum approach distance was 4 km from the crater and the hazard status remained yellow.

2 July: Activity remained stable. The number of exhalations decreased; all of low to moderate intensity, some were accompanied by steam and gases.

3 July: Bad weather and clouds limited visibility.

4-5 July: Short exhalations of low-to-moderate intensity; some accompanied by steam and gases.

6 July: Except some isolated low intensity exhalations, the activity remained stable and at low levels. In the morning a small steam-and-gas plume blew SW.

7 July: Seismicity remained low and only small isolated exhalations were recorded. Small steam-and-gas plume.

8 July: Generally low activity. Seismic signals indicated that only a few, moderate exhalations took place. In the morning a gas and steam plume rose ~700 m above the crater and then dispersed .

9-11 July: Intense cloudiness and bad weather.

12 July: Low activity. Small and isolated exhalations, some accompanied by light steam-and-gas puffs. A small steam-and-gas plume was observed all day, blown W.

13 July: Activity increased slightly in the morning at 0931. A moderate exhalation produced a small but persistent ash emission directed to the W. Emissions and tremor continued until 0950.

14 July: Stable, low activity. Small gas plume blew W.

15 July: A few moderate exhalations were recorded seismically and were accompanied by gas-and-steam emissions. Some signals indicated rockfalls. In the morning only a small gas plume blew W .

17 July: Although cloudiness obscured the volcano most of the day, in the morning a steam-and-gas plume blew W.

18-21 July: In the morning on each day a dense steam-and-gas plume blew W.

22 July: Seismic signals revealed isolated, short exhalations; some contained ash. The latter was seen in the morning at 0743. The ash produced was light; it rose up to ~1 km above the summit and rapidly dispersed SW.

23-28 July: Seismic signals indicated isolated, short exhalations; some were accompanied by gas and steam. In the morning a dense steam plume blew W.

Activity during 13-14 August. Servando De la Cruz-Reyna provided this description of a mid-August earthquake followed the next day by an eruptive outburst. At 1447 on 13 August a M 3.9 volcano-tectonic earthquake took place at a depth of ~12 km beneath the central part of the volcano (~6.5 km below sea level). During the earthquake, several tiltmeter stations recorded a step-like down-going displacement. Two smaller earthquakes occurred at various depths beneath the edifice that day. That night some low-amplitude, harmonic tremor signals were detected between 2200 and 2400; afterwards seismicity declined.

At 1850 on 14 August a moderate ash emission lasting about 15 minutes produced a column that rose to about 4-5 km over the summit. A low velocity wind (10 km/hour) distributed very light ash falls on some towns in the NW sector of the volcano about two hours later. On August 16, at 2149, a similar event produced a 2-3 km high column over the summit. The pattern of light ashfall repeated. Afterwards, Popocatepetl volcano returned to the previous low-level of activity prevailing since May 1998.

Technology versus rumors. Besides a rapidly growing web site and a broad network of seismic and tilt stations, CENAPRED has also adopted other innovative approaches. For example, a near real-time image of the N summit area is transmitted via microwave linkage and can be viewed on the CENAPRED web site at two resolutions. An infrared camera discloses thermal signatures of erupting plumes.

Still, despite these advances in monitoring technology and communication, during mid-1998 members of the lay public became increasingly concerned about rumors of doomsday scenarios involving Popocatépetl, some of which were broadcast via the media. In response, in June 1998 Roberto Meli, the Director General of CENAPRED, posted an informative note on their web site. He addressed the rumors and explained that there was an absence of scientific evidence for substantive changes in the volcano's behavior in the near future.

Following moderate eruptive outbursts in mid-August (BGVN 23:07), Popocatépetl returned to the low-level activity that has prevailed since May 1998. A few minor increases in ash emissions during subsequent weeks were noted by CENAPRED in the daily internet log.

Frequent moderate emissions took place on 24 August. The largest events occurred at 0402 and 1215. A small plume from a steam fumarole drifted to the SW. At 1553 a tectonic earthquake of magnitude M 2 occurred, and was located 2.5 km SE of the crater at a depth of 3.7 km. At 1613 40 minutes of high-frequency tremor of variable amplitude began, accompanied by the emission of ash, gas, and steam in a column rising 1,500 m above the summit. Activity diminished to normal by the next day.

Isolated, moderately large exhalations of short duration were noted on 28 August. Some of these exhalations were accompanied by small puffs of ash, steam, and gas. At 1807 a high-frequency tremor signal was recorded followed 2 minutes later by mild explosions lasting 8 minutes. Visibility was limited throughout the day by intense meteorological cloudiness, but presumably the explosions caused a gas, steam, and low-density-ash column that was blown NE. After this event activity returned to previous levels.

A slight increase in activity early on 31 August consisted of several moderate, but short, exhalations and some small high-frequency tremor episodes. The largest occurred at 0418 followed 6 minutes later by an exhalation. The tremor continued for about 15 minutes. At that time glow from inside the crater could be seen on the summit video camera. During the night several rockfalls were recorded.

After several days of low activity, a moderately large exhalation occurred at 0539 on 8 September. Its intense phase lasted 6 minutes, followed by a 15-minute-long tremor. Although the volcano was obscured by cloud, Doppler radar indicated ash close to the crater (figure 26). Reports were received of slight ashfall to the SW at Huequechula, Puebla. At 0844 another exhalation, much smaller than the one recorded earlier, lasted only 2 minutes; no ash emission was detected. At 1439 a low-frequency tremor signal started and persisted until 2100. Although scientists suggested that this signal might herald more intense and explosive exhalations in the next hours or days, similar to last April, activity decreased the next day.

Figure 26. Radar image of an eruption plume from Popocatépetl taken at 0548 on 8 September 1998. The source of the radar is at CENAPRED headquarters located at the center of the circles. The volcano is 60 km SE at the right side of the image. The ash cloud is identified by a gray spot close to the volcano. Image courtesy of CENAPRED.

Following a large ash exhalation on 8 September (BGVN 23:08), eruptive activity at Popocatépetl decreased in intensity and duration. CENEPRED reported a few moderate emissions during September that caused local ashfall.

Small-volume, discrete, short-duration emissions containing ash, sometimes accompanied by steam and gas, were recorded occasionally during the period 9-15 September. Brief episodes of harmonic tremor were also recorded. During the night of 14 September glow reflected from clouds over the crater was seen.

Moderate exhalations of steam, gas, and light ash took place during 16 September. Several brief episodes of high-frequency tremor were recorded that afternoon; the largest emissions occurred at 1546-1552, 1604, and 1611. Ashfall was reported at Amecameca, 20 km NW of the volcano. Despite bad weather that reduced visibility most of the day, a dense column of steam and gas was seen rising 700 m above the summit before being blown to the NW. Activity decreased to stable background levels on 17 September. A dense steam and gas cloud seen on the morning of 18 September dispersed to the NE; as the cloud gained altitude, its direction changed to the south. SO2 measurements showed significant increases following the 16 September explosion over levels earlier in the month.

Another moderate increase in eruptive activity began a few days later. A steam and gas column rising 1 km above the summit was observed during 20 September. Brief, moderately intense emissions of steam and gas, sometimes with light ash puffs, took place throughout the morning of 21 September. An explosion at 1148 that morning produced light ashfall in towns up to 20 km NW of Popocatépetl. A similar but less intense event occurred at 1543. Emissions decreased to relatively low levels until 1225 on 22 September when a moderate explosion lasting 7 minutes produced a steam, gas, and ash plume that rose 4 km above the summit. Visibility during 22 August was poor due to bad weather, but a large ash cloud near the crater was detected by Doppler radar. Ash was dispersed during the afternoon NW of the volcano, producing light ash falls in the suburban SE of metropolitan México City.

Following the explosion on 22 September, eruptive activity paused until a similar explosion occurred at 1829 on 23 September. This explosion lasted 6 minutes and produced a 3-km high column of steam, gas, and ash. Ash fall was reported in towns SW of the volcano. Eruptive activity soon decreased again, stabilizing at low levels of small, isolated emissions of steam and gas, typical of earlier in September. An exhalation at 1025 on 24 September was followed by 30 minutes of low-frequency harmonic tremor. An A-type earthquake of M 2.1 located 1.8 km E of the crater at a depth of 3.9 km was recorded at 2224 on 24 September, and another moderate exhalation lasting 7 minutes began at 2332.

There were a few instances of moderate disturbance during October, and a relatively large emission occurred on 17 October; otherwise, Popocatépetl remained generally stable at low levels of eruptive activity, including almost daily emissions of steam and gas. Since the possibility of explosions remained, authorities recommended that no one approach within 4 km of the crater. The caution light remained "yellow" throughout the month.

Steam-and-gas fumaroles rose up to 500 m above the summit several times during the first week of October. The emissions usually blew SE. Two slightly larger exhalations lasting 5 minutes each at 0218 and 1409 on 4 October may have also released ash, but this was unconfirmed owing to bad weather obstructing views of the volcano. At 2312 on 5 October an explosive event began. An intense two minute phase was followed by 30 minutes of steam, gas, and ash emission that formed a plume 4 km above the crater. Glow was also seen at this time. Activity quickly diminished to previous low levels.

At 1715 on 17 October a larger exhalation began: its intense phase lasted about 16 minutes and produced an ash column (figure 27). The plume rose 2 km above the summit and blew NW (towards Mexico City).

Figure 27. Basal portion of an ash column from Popocatepetl on the afternoon of 17 October as seen from a video monitor. Courtesy of CENAPRED.

The ash column was initially detected by Doppler radar located at CENAPRED headquarters in Mexico City, and staff there immediately informed air-traffic controllers. The ash emission persisted for 20 minutes, after which the volcano returned to its previous low-level activity (steam and gas emissions only). One hour after the beginning of the event, reports were received of ashfall at Amecameca, Tenango del Aire, and other towns NW of the volcano.

At 2040 another smaller exhalation took place with a duration of only 1 minute. At about 2100 light ash from the earlier eruption fell at CENAPRED headquarters, UNAM, and at other places in SW Mexico City. Activity soon dropped to characteristic low-intensity exhalations. A similar moderate emission lasted 1 minute at 1859 on 24 October; the event was followed by low-amplitude, high-frequency tremor for about 20 minutes, producing a 2,500-m-high column of gas, water vapor, and ash.

A-type earthquakes were recorded at 0956 on 16 October (M 2.6, at a point 6.6 km below the summit), at 2227 on 22 October (M 2.0, at a point 7 km below the crater), at 1751 (M 2.1) and 1919 (M 1.8) on 29 October, and at 0942 (M 2.4) on 30 October. Two minutes of low-amplitude, low-frequency tremor began at 1355 on 29 October. None of these events seemed to affect activity at the volcano.

A change in the typical low-level steam-and-gas emission regime in late November and early December suggested that a new lava body was growing inside the crater. The following has been condensed from CENAPRED bulletins.

Low-level activity continued during the first three weeks of November, and included low-intensity, short-duration exhalations of steam and gas with occasional eruptions of ash. Bad weather obstructed observations on many days. Authorities recommended that no one approach within 5 km of the crater because of the danger of sudden explosions. The volcanic alert level remained at yellow, indicating a state of heightened caution. Several A-type earthquakes occurred (on 6, 14, 15, 16, and 17 November; M 2.1-2.9), generally 3-4 km E or SE from the crater, none of which seemed to affect eruptive activity. One exceptional emission occurred at 0109 on 9 November; its intense phase lasted one minute and was followed by 12 minutes of high-frequency tremor.

At 1753 on 19 November a moderately large eruption was followed by five smaller ones. The series lasted seven minutes and produced an ash column that rose 2-3 km above the summit and dissipated NNW. Light ash fall was reported in the neighboring town of Amecameca. At 2019 a slightly smaller exhalation lasted nine minutes.

At 1302 on 22 November the volcano began a substantial increase in activity, starting with a sequence of small ash emissions; light ashfall was reported at Paso de Cortés and Amecameca. This activity continued into the night with about 40 separate emission events by midnight. Exhalations increased, and at about 0430 on 23 November harmonic tremor episodes were recorded. At 0530 incandescence at the crater could be seen, at 0854 high-frequency tremor started, and at 0922 a moderate ash emission generated a column 3 km above the summit. By noon about 100 exhalations had been recorded. Dense fumarolic clouds of gas and steam were blown NW. Beginning at 1245 activity increased again: high-frequency tremor and emissions occurred at a rate of one per minute. Although the summit was obscured by cloud, it was assumed, based on reports from local towns, that ash emissions were continuous. After 1515 seismicity increased to saturation levels on most of the recording instruments. Later, an emission of steam, gas, and ash could be seen. At 1630 seismicity started to decrease.

Small, low-frequency tremor signals began around 0200 on 24 November, and intensified between 0300 and 0600. The tremor was accompanied by continuous emissions of gas, steam, and some ash, blown to the SW. At 1257 another increase of activity began. Low-frequency tremor of variable amplitude was recorded until 1600. Poor visibility prevented direct observation of the summit during most of the day.

A steam plume that rose 2-2.5 km over the summit persisted until 0803 on 25 November when a moderately large explosion lasting one minute produced an ash plume that rose 3-4 km over the summit (figure 28) and threw rock fragments to a distance of 2 km. The top of the plume moved N, while the lower part moved SW; ashfall warnings were issued to towns in those directions. A low-frequency tremor signal followed the explosion and persisted through the day. Other explosions occurred at 1205 and 1658 on 25 November. Although the explosions were heard in nearby towns, there were no reports of large ash emissions, and it is likely that the ejected rock fragments were dispersed around the crater.

Figure 28. Characteristic explosions and associated plumes from Popocatépetl taken by CENAPRED's video monitor during 25-26 November 1998. Scenes 1-3 (top) are from 0803-0805 on 25 November; scenes 4-5 (bottom) are from 1013-1015 on 26 November. Courtesy of CENAPRED.

An increase in tremor was followed by new explosions at 0654 and 0719 on 26 November. Moderate steam-and-ash plumes rose to a height of 1,500 m above the summit. A stronger exhalation at 0931 produced a moderate plume of steam and ash rising 3-3.5 km above the summit. Other explosions at 1013 (figure 28) and 1104 produced higher ash columns. In all cases warnings were issued to air-traffic controllers. A new warning to the general population recommended approaching no closer than 7 km from the crater. Tremor was followed by volcanic earthquakes at 2113 and 2220; both events produced moderately large explosions and ash plumes, and during the later event incandescent lava fragments were thrown to a distance of ~1.5 km.

Moderate explosions were detected in the crater at 1206, 1333, 1749, and 2345 on 27 November, and at 0242 and 1021 on 28 November. All of them, except the third, expelled incandescent fragments of lava around the crater to a distance of 0.5-2 km, and produced moderately large emissions of ash, rising in most cases up to 4 km over the summit. This activity was detected against a background of low-level exhalation and tremor signals of decreasing amplitude. Light ashfall had been reported in Tlacotitlán at 0130 on 28 November. During 28 November activity increased again following several short harmonic tremor signals at 2130. At 2228 a moderate volcano-tectonic event was followed by small tremor episodes.

At 0002 and 0305 on 29 November two explosions were preceded by low-frequency tremor. The second explosion produced a shock wave clearly heard at Paso de Cortes and San Nicolás de los Ranchos. Large quantities of glowing rocks ejected from the crater could be seen falling in a area of ~3 km radius. There was also a large ash emission. At 0654 a moderately large emission, lasting seven minutes, formed an ash plume 4 km above the summit. At 1118 there were several low-frequency harmonic tremors. A moderately large explosion at 1645 ejected incandescent lava blocks around the cone and produced an ash plume up to 7 km above the summit (according to personnel working close to Paso de Cortes).

Tremor episodes and moderate emissions of steam, ash, and gas with occasional explosions persisted over the next week. One explosion at 0929 on 30 November began with a strong shock wave and blast, ejected fragments over its flanks 2-3 km from the crater, and produced an ash column 4 km above the summit. At 1853 on 3 December an explosion ejected incandescent fragments over the SE flanks and produced a moderately large ash cloud, carried by the wind to the SE. The explosion signal lasted one minute, followed by 15 minutes of tremor. At 1255 on 4 December an explosion threw hot debris on the SE flanks and produced an ash plume that rose 4-5 km above the summit. Another explosive eruption at 1511 on 6 December ejected incandescent rocks over the E and N flanks and produced an ash column 5 km above the summit that dispersed to the NW. This event lasted 1.5 minutes and was followed by high-frequency tremor for four minutes. Three explosions were recorded on 7 December at 0241, 0449, and 0623; glowing fragments fell on the E and N flanks and an ash column rose 4 km. The last of these events lasted 1.5 minutes and was followed by high-frequency tremor for 10 more minutes. During 8 December frequent exhalations with durations of 3-10 minutes each produced steam-and-ash columns 2 km above the summit.

Activity became stable at lower levels during the second week of December, persisting until the time of this report (15 December).

Weather clouds obstructed visibility during much of December. Scientists and civil authorities continued to recommend that no one get closer than 7 km from the crater. The hazard status remained Yellow.

At 0140 on 9 December a five-minute series of explosions ejected incandescent fragments over the flanks. These explosions were preceded by tremor and an A-type earthquake of low magnitude. At 0929 on 10 December another explosion ejected rocks onto the E flank (figure 29) and produced an ash column ~4 km high. High-frequency tremor was associated with the emission of gas, steam, and some ash plumes rising several thousand meters. An M 2.7 earthquake, 2 km SW and 11 km beneath the crater, occurred at 1839 on 13 December but did not affect eruptive activity.

Figure 29. Series of images showing the N flank of Popocatépetl from a monitoring camera taken at 0931-0932 on 10 December. Courtesy CENAPRED.

On 15 December the instrument station at Canario, on the N flank, went out of operation due to the intense eruptions of recent weeks. At 1750 on 15 December an explosion lasting one minute ejected incandescent fragments over a radius of 2-3 km. The explosion also produced an ash plume 3-4 km above the summit.

After an explosion at 1847 on 17 December activity fell immediately, with only isolated low-intensity exhalations and periods of high-frequency, low-amplitude tremor. According to field and aeronautical reports, the ash column reached 4-5 km above the crater and slowly dispersed ENE. Glowing fragments that fell on forested and grassy areas produced fires that persisted into the night but did not present hazards for nearby towns.

Following several A-type events, a moderate exhalation occurred at 2010 on 20 December. This event produced ash carried NW before falling over the airport in México City. Flight operations were closed between 2330 and 0115. Several hours of high-frequency, medium-intensity tremor were recorded during 24 December. At 0044 on 31 December an A-type earthquake took place with a magnitude of 3.5 and depth of 12 km under the summit.

Activity at Popocatépetl during January was generally stable, characterized by periodic low-level, short-duration exhalations and high-frequency tremor. Restrictions on access closer than 7 km to the crater continued. The traffic-light alert signal remained Yellow.

Volcano-tectonic A-type earthquakes occurred at: 1302 and 2327 on 1 January; 0126, 0550, and 0745 on 9 January; 0152 on 10 January; and 2353 on 14 January. Twelve minutes of tremor began at 1127 on 10 January and 40 minutes of tremor were recorded on 20 January beginning at 0634. Several moderately large exhalations of steam and gas were noted during the month including those on 4, 9, 10, and 15 January. A small ash puff rose 500 m above the summit on 18 January before dispersing to the SW. Emissions of steam, gas, and ash were accompanied by episodes of tremor on 20, 21, and 26 January.

Low-amplitude, high-frequency tremor was detected beginning at 1306 on 27 January. The public warning against approaching closer to the crater than 7 km was reemphasized. An increase in the amplitude of the tremor signal began at 1341. The increase was accompanied by a plume of steam, gas, and ash that rose 3 km above the summit. Prevailing winds carried the ash toward the NE where minor ashfalls were expected. Activity returned to previous lower levels early in the evening. A similar episode occurred on 29 January. Fifteen minutes of tremor began at 1254 accompanied by a steam-gas-and-ash plume that rose 3 km above the summit before being blown to the NE. More moderate activity returned later in the day.

In response to some possibly incorrect information contained in a local news broadcast on 14 January, CENAPRED outlined changes to the crater as observed by scientists on the volcano. No significant changes have been observed since the explosive activity of 17 December 1998 (BGVN 23:12). That explosion removed a moderate amount of lava from an inner craterlet on the dome inside the main crater. This inner craterlet was ~300 m in diameter before 17 December and, after the explosion, it had a depth (relative to the dome) of ~75 m. These dimensions are nearly identical to those it had in October 1998. The shape, conditions, and general activity within the main crater are similar to those in October.

Frequent low-intensity short-period exhalations, with occasional ash plumes and incandescent ejecta, and periodic low-frequency tremor episodes characterized activity at the volcano during February and March. Due to the possibility of explosions, authorities continued to recommend that no one approach closer than 7 km to the crater. The volcanic alert status remains yellow.

Beginning at 0315 on 2 February a low-frequency tremor fluctuating in amplitude was recorded; it persisted for about 24 hours. During the next few days low-intensity events produced small steam, gas, and light ash emissions. Some high-frequency tremors were recorded on 9 February beginning at 2225 and lasting 14 minutes. Also on 9 February, three tectono-volcanic earthquakes occurred (at 0845, 1151, and 1152) SE of the summit at distances of ~8 km. These ranged from M 2.0 to 2.5. Low-frequency harmonic tremor was recorded on 12, 17, and 21 February. An A-type M 2.8 earthquake occurred at 2000 on 18 February.

Small exhalations preceded by low-frequency, low-amplitude harmonic tremor continued in March. "Pulgas" (small pulsating signals) were recorded; occasionally these preceded explosive events by a few hours. On 1 March small emissions of incandescent fragments were contained within and, later, an ash plume rose 1,000 m above the summit before moving E. Pulgas and low-frequency harmonic tremor of variable amplitude continued intermittently for a few days. The shape of recorded signals suggested a growing lava dome in the crater. During a flight on 4 March to measure CO2 volumes, gas and light ash could be seen emanating from the crater. On 8 March a vertical ash plume rose 5 km above the summit before being blown NE and, at a higher altitude, SW. Light ashfall was recorded E of the volcano.

Good weather on 8 March permitted several close-up images to be taken with the video camera. These images showed the glacier partially covered with ash and several impacts from explosions in December 1998. Small flows of melting snow or ice were seen.

At 1940 on 11 March a moderate explosion ejected incandescent fragments over the N flank. Radar showed an ash plume directed to the NE. At 2211 on 12 March another moderate explosion ejected incandescent fragments over the NE flank to a distance of 500 m. A larger explosion at 0531 on 18 March sent incandescent fragments over the E and SE flanks in a radius of 3 km. Poor weather obscured the summit on 19 March when another large explosion occurred. Ash was initially detected by radar and at 1845 an airplane reported a plume 16 km NE from the crater. The explosion was heard and felt in the town of Amecameca.

During April 1999 the volcano returned to low levels of activity. Small sporadic exhalations occurred that occasionally carried sufficient ash to be visible on Doppler radar.

At 0031 on 2 April an A-type earthquake of M 2.1 occurred at a depth of 7.6 km centered 3 km NE of the crater. Small ash emissions were accompanied by gas and steam. On 3 April a fumarolic emission with some ash could be seen descending the NE slope.

A moderate explosion, lasting 40 seconds in its most intense phase, began at 0327 on 4 April. People in the town of San Andres Calpan, 20 km from the volcano, heard the explosion and observed incandescence over the crater. The incandescence was also recorded by CENAPRED video cameras, which showed that during the event incandescent material was ejected over the E flank. Doppler radar recorded an ash emission following the explosion. Activity soon returned to a more stable condition. At 1240 an A-type earthquake of M 2.4 occurred 8 km NE of the crater at 6.2 km depth, and at 0945 on 5 April an A-type earthquake with M 2.2 occurred 8.5 km NE of the summit at 6.6 km depth.

Monitors detected a moderate exhalation lasting 90 seconds beginning at 0031 on 11 April. This event was followed by six similar exhalations during the next 18 minutes. Doppler radar did not detect any significant ash emission, and no incandescence was observed in the crater. During 14-15 April small and medium exhalations with durations of 1-4 minutes were accompanied by vapor, gas, and some ash emissions. At 1056 a moderate explosion lasted ~4 minutes and produced a 3.5-km-high ash cloud that was transported NE.

Earthquakes were recorded near the volcano on 26 April. The first started at 0014 with M 2.2, located 9 km SE of the crater at a depth of 4.3 km. Another event occurred at 0954 with M 2.4 located 8 km SE of the crater at a depth of 3.4 km.

During May, Popocatépetl generally displayed low activity. Seismicity included small, isolated exhalations of gas, steam, and ash. The alert status remained at "Yellow" with a 7-km radius of restricted access.

A gas-and-steam exhalation at 1315 on 5 May lasted 12 minutes and produced a plume to ~1 km above the crater. Gas-and-steam exhalations at 1435 and 1623 on 6 May lasted 30 and 14 minutes, respectively, and both produced plumes rising ~1 km above the crater. Separate seismic events at 2053 and 2133 on 10 May were possibly rockfalls on the S flank. Visibility was obstructed by clouds. The next morning a steam column from the crater was seen rising about 500 m above the summit. Beginning at 2200 on 13 May, a sequence of seismic events with variable amplitudes was accompanied by high-frequency tremor. Stable seismic conditions returned after about an hour.

Activity increased on 15 May and included small rivulets of meltwater. At 0246 on 16 May a moderately large explosion occurred. Later (at 0706) a moderate exhalation, lasting three minutes, produced an ash column rising 2,500 m above the summit before dispersing to the SW. No ashfall was reported. Several volcano-tectonic events were recorded on 17 May; their signals possibly related to small rivulets of meltwater descending the N flank.

During the night of 23 May a swarm of three tectonic earthquakes was centered 2.5 km E of the crater. The first, at 2251, had M 1.7 and was located at a depth of 4.5 km; the next occurred at 2338, had M 1.9, and was located at a depth of 5.2 km; and the last was at 2339, had M 2.0, and was located at a depth of 5.8 km.

The low level of activity displayed in May continued, with small exhalations and minor fumarolic emissions until 12 June. However, seismic activity increased on 12 June and continued for the next 10 or 11 days. At 1209 and 1600 on 12 June, two M 2.2 volcano-tectonic events occurred under the crater and SW of the volcano. At 1542 on 15 June, a large earthquake (M 6.7) centered between the states of Puebla and Oaxaca did not affect the volcano. Bad weather had obstructed visibility earlier, but that afternoon observers saw small fumarolic emissions of steam and gas.

Seismicity increased on 16 June as several volcano-tectonic events were recorded in the morning, most with magnitudes between 2.5 and 3 and two larger ones with M >3. These events were located 4-7 km below the summit crater. The last event occurred at 0206 on 17 June. This seismicity did not produce any important external manifestations except a small exhalation on the morning of 17 June accompanied by a light ash puff blown to the W.

On 21 June two earthquakes in Guerrero did not effect the volcano. No other events were reported for the month and by 30 June the radius of restricted access was reduced to 5 km from the 7 km previously recommended.

Low-level activity continued throughout most of July, August, September, and into the first week of October, with only small-to-moderate exhalations and some light gas and steam emissions. Generally, fumarolic activity was low, but clouds frequently obstructed visibility. The hazard status remained Yellow and the radius of restricted access remained at 5 km. A M 7.4 earthquake in the state of Oaxaca on 30 September did not affect the volcano.

Low-magnitude microseismic and/or tectonic events occurred occasionally. Type-A earthquake events were recorded at the following times: M 2.2 at 0141 on 14 July (preceded by a type-A microseism); M 3.3 at 2053 on 15 July; M 2.1 at 2336 on 23 July (followed by a small tectonic type-A event on 25 July); a small-magnitude event at 1638 on 29 August; and two events at 2008 and 2148 on 1 September of M 2.2 and 2.5, respectively.

Several low-magnitude tectono-volcanic earthquakes were also detected as follows: M 2.7 at 0654 on 28 July; two events at 2029 on 29 July with M 2.0 and 2.6, respectively; a M 2.5 event at 1431 on 6 September at a depth of 7.9 km from the summit and 5 km S of the crater; M 3.2 at 2047 on 8 September with its hypocenter at 7.1 km below the summit and 6 km S of the crater; and another M 3.2 event at 0834 on 27 September at a depth of 5.3 km under the summit and 6 km SSE of the crater.

Moderate exhalations starting in late August continued through September and into the first week of October. At 0920 on 27 August two small ash emissions caused light ashfall over several towns on the W flank. Another emission on 1 September caused minor ashfall. A larger event with a duration of two minutes occurred at 2205 on 5 September, causing light ashfall over several towns. At 0757 on 20 September a small exhalation ejected a plume 1 km above the summit before dispersing to the W. Two moderate exhalations occurred at 0916 and 0949 on 29 September, both lasting about 2 minutes, with ash falling W of the volcano about an hour later.

Volcanic activity during the first week of October, subsequent to a M 7.4 earthquake in the state of Oaxaca on 30 September and a number of aftershocks, remained similar to recent months. At 1101 on 3 October, a moderately large exhalation lasted for more than 15 minutes; the ash column rose to 4 km above the crater and ash fell on several towns to the SW.

This report covers the period from early January to 26 February 2000. After five months of quiescence, in October 1999 the monitoring system detected a slightly increased rate and intensity of exhalations. Low-level deformation began in early January 2000. Data from GOES hot-spot monitoring also indicated increasing thermal activity at the volcano crater.

In February 2000, the exhalation rate increased and low-level harmonic tremors began, suggesting the beginning of a new dome growth episode. These conditions led to an increase in alert status, from Yellow-1 to Yellow-2. Overall, the increased levels of activity were lower, however, than those detected in 1997 or 1998.

On 25 February, observers on a helicopter flight looking into the summit crater confirmed the presence of a small blocky lava dome. The new dome was growing at the center of the crater produced by the explosions that destroyed the previous dome during late 1998-early 1999. The new dome's diameter was slightly over 50 m, and it stood about 11 m high. The surface of the dome appeared to be formed by large light-gray blocks. A portable thermal scanner in the helicopter targeting the center of the dome gave a maximum temperature of about 340°C. The lava production rate around this time seemed to be at least one order of magnitude lower than in the previous episodes.

This report covers the period form 15 June to 22 August 2000. The highest ash column in this period rose to over 5 km above the summit.

Throughout most of the reporting period, activity remained stable with periodic exhalations of small amplitude and duration. However, two small mudflows were reported: one on 23 June and the other on 24 June. According to CENAPRED, the mudflow on 24 June did not reach any human settlements. No information was available concerning the 23 June mudflow.

On 3 July, two small exhalations generated ash clouds that reached 1 and 2.5 km above the summit and ash fell over the volcano's SW sector. On 4 July, ash from a small exhalation fell in Tetela, a town ~15 km SW of the crater. On 14 July, the volcano erupted and produced an ash cloud that reached 1.6 km in height. According to the Associated Press (AP), the ash from this eruption was blown N and did not significantly impact any populated regions surrounding the volcano.

On 4 August, two closely spaced explosive eruptions occurred. The first at 1251, a moderately large exhalation, lasted 2 minutes. The second one occurred at 1255 and lasted 1.5 minutes. The resulting ash cloud rose to greater than 5 km above the volcano. Ash reportedly fell in nearby communities (Atlautla, San Juan Tehuixtitlan, San Pedro Nexapa, Amecameca, and Tenango).

At 0910 on 10 August, Popocatépetl erupted again. Ash reached to 3.5 km above the volcano. The ash clouds traveled to the W. A second eruption was visible in GOES 8 imagery. It was expected that nearby Mexican states would be coated with a thin layer of ash. At 19:15 on 23 August, a moderate exhalation produced ashfall in the nearby communities of San Pedro Nexapa and Amecameca (~12 km NW and ~16 km NW of the summit, respectively). Throughout the rest of the reporting period there were exhalations of low intensity and short duration that mainly involved gas with small amounts of ash.

Several volcano-tectonic earthquakes, ranging in magnitude from 1.7 to 2.3, occurred during the month of July. The first of these was on 2 July. It was followed by earthquakes on 6, 8, 9, 11, 15, and 23 of July. Three volcano-tectonic earthquakes occurred on 20 July, all under M 2.5. On 1 August, three more tectonic earthquakes were recorded, M 1.9 - 2.7. Other earthquakes occurred on 5 and 10 August; both were less than M 2.

Popocatépetl's volcanic hazard level remained at yellow. CENAPRED recommended that all visitors remain 7 km or more from the crater.

Increased gas-and-ash emissions in early November elevates hazard status

This report covers the period from 22 August to 4 November 2000 (table 13). Volcanic activity during this period generally was similar to that for the preceding summer months (BGVN 25:07). Steam-and-gas exhalations occurred frequently, with daily variations in intensity and duration. The reported number of daily exhalations varied from only a few to as many at 140 (12 September). Some exhalations included ash, several of the larger ones generated ash plumes 5 or 6 km above the crater and caused ash falls on towns downwind, including NW-flank towns. On 4 September ashfall reached the S limit of the capital. Seismic tremor was also common, both harmonic and high frequency, some of low amplitude and duration and others lasting from 30 minutes to several hours.

Table 13. Summary of Popocatépetl's activity during 22 August to 4 November 2000. Exhalations were seismically detected. The symbol x means tremors were reported (for stated duration). Courtesy of CENAPRED.

Volcano-tectonic (VT) earthquakes registered on 29 August; 4, 6, 7, 17, 22, 23, September; 16, 22, 27 October; and 2, 3 November. Most of these were M <=2.0 but those occurring on 22 September, 16 October, and in early November were M 2.4-3.1. The latter two periods also coincided with explosive exhalations as described below.

On 16 September, a small dome (very small compared to the one that grew in 1997) was observed during a flight over the crater. On 18 September, an SO2 level of 12,700 tons/day was registered. The VT event on 22 September was registered 20 km SE of the volcano. An explosive exhalation at 0520 on 16 October sent incandescent fragments ~1 km to the NE. As noted above, 4 VT earthquakes preceded this exhalation, one (the largest) was M 2.4. A second explosive exhalation occurred later that day at 1142 although no incandescent fragments were confirmed.

On 29 October, 102 exhalations registered and, at 1710, a moderately large exhalation produced an ash plume that rose to 3 km above the crater and moved ~25 km WNW. On 2 November, the volcano had an increase in activity and at 1717 there was a moderate exhalation followed by 30 minutes of high-frequency tremor. The event sent an ash plume 3 km above the crater; ash fell on several towns. At 1827 a VT event of M 3.1 was registered SE of the volcano. Then, between 1300 and 1630 on 3 November, four VT events occurred; these M 2.2-3.0 events were located below the crater. Later that day, the Scientific Committee discussed the increased activity over the previous few days and, in conjunction with the Civil Protection Authorities, decided to change the hazard status to Yellow III, recommending further that the security radius around the volcano be increased to 10 km (from the radius of 7 km that had prevailed for many months).

December set records in tremor, dome extrusion rates, SO2 flux, and tilt

Around the end of the year 2000 and in January 2001 Popocatépetl extruded dome lavas at record-setting rates and amassed the largest active dome ever recorded on the volcano. The seismic energy released in one 25-hour interval in mid-December was greater than the accumulated energy for any entire year for which measurements are available. The highest plume of the interval rose to ~8 km above the summit crater.

During late 2000 into January 2001, but particularly in December, tremor reached the biggest amplitudes yet recorded during this multi-year crisis; it was felt by people 12-14 km distant, and one tremor episode prevailed for ~10 hours. Another episode saturated instruments to the point of damage and drove tiltmeters in dramatic oscillations.

Although impressive plumes had been seen before in this crisis, for the first time hot ash and gases began escaping the summit crater regularly, accompanied by pyroclastic flows and mudflows. The longest pyroclastic flow reached a runout distance of ~8 km. Preliminary photo analysis made during episodes of harmonic tremor in mid-December led to lava extrusion-rate estimates that were more than an order of magnitude higher than those typically seen at stratovolcanoes. During mid-December, sulfur dioxide (SO2) fluxes reached ten to twenty times larger than the volcano's typical ~5,000 tons/day.

Although later Bulletin reports will provide more details, what follows here are critical highlights for assessing the behavior through 29 January. The report was provided by Servando de la Cruz-Reyna, Carlos Valdés-Gonzalez, Roberto Quaas-Weppen, and affiliated CENAPRED scientists noted below.

Relative quiet followed by unrest. The previous episode of dome growth took place in February 2000, resulting in the smallest of all domes grown since 1996 (see BGVN 25:01). After a period of relative quiescence, unrest followed at Popocatépetl in early September 2000 (BGVN 25:10). This marked the beginning of a new episode.

September unrest was marked by two seismic observations. First, harmonic tremor appeared in the peak of the exhalation signals. Second, tectono-volcanic earthquakes below the crater were followed by long-duration explosive eruptions that generated higher-altitude plumes. GOES satellite imagery depicted strong thermal anomalies in the crater. Still, only comparatively minor dome growth was detected in mid-September, and this same pattern continued during early November. A variable, somewhat reduced level of activity continued into early December.

Escalation in December 2000. As discussed below, RSAM values climbed precipitously during a 7-day interval in mid-December. Prior to that, on 2 December an ash emission of moderate-to-large size lasted about 90 minutes. On 6 December, nine low-magnitude earthquakes (M ~1.7-2.4) occurred followed by a similar swarm on 8-9 December. These earthquakes, in turn, were followed by a period of low-frequency harmonic tremor that lasted about 5 minutes. Although brief, this tremor had the largest amplitude recorded since this eruptive period began in 1994.

Particularly during December, tiltmeters, for the first time since their installation, registered all of the large tremor signals (figure 30). Tilt oscillation amplitudes were typically in the range of 100 µrad, reaching peak-to-peak values near 200 µrad. Seismicity during 11-18 December was extremely high (figures 31 and 32).

Figure 30. Tilt at Popocatépetl recorded by various stations for 11-20 December 2000. The large tilt displacements occurred in conjunction with high-amplitude tremor. The x and y directions are neither radial nor strictly tangential in orientation. The two axes lie at right angles on a horizontal plane such that a line 45 degrees away and bisecting both these axes trends through the center of the volcano. Courtesy of CENAPRED.

Eruptive activity increased on 12 December 2000 with frequent ash-bearing emissions (up to 200 per day), some of them reaching about 5-6 km above the volcano's summit. During the following night observers saw incandescence and small amounts of hot debris. Similar activity and longer-duration eruptions during 13-15 December produced light ashfalls on towns around the volcano.

Early on 15 December more episodes of high-amplitude, low-frequency harmonic tremor were detected, lasting a few minutes. At 1404, the low-frequency harmonic tremor grew to a continuous signal, with amplitudes peaking on all the monitoring stations, including the most distant one. These signals were strong enough to be felt by residents 12-14 km away, and to be detected at stations of the Mexican Seismological Network as far as 150 km from the volcano. This tremor episode remained at constant intensity for about 10 hours, and may have stemmed from very high rates of lava extrusion.

Starting early on 16 December activity underwent a dramatic drop that was reversed 16 hours later by a return of low-frequency harmonic tremor of increasing amplitude. This tremor again saturated all monitoring stations; it lasted about 9.5 hours. The amplitudes of the signals were so high that pen drivers and several styli of the paper-drum recorders were damaged. A still-larger tremor episode took place on 18-19 December.

Figures 31 and 32 illustrate the seismic traces and cumulative RSAM data. RSAM peaked during an interval of slightly over 7 days in mid-December, when low-frequency tremor prevailed for ~25 hours and the seismic energy released exceeded that of the rest of the year 2000. Actually, the peak surpassed that accumulated during any previous entire year for which records exist (including 1997, see figure 32).

Figure 31. Paper-drum records from Popocatépetl photographed while laid out on a flat surface. The records depict the record-setting seismic signals at Canario station (PPPN) on 30 June 1997 (labeled "a") and on 18-19 December 2000 (labeled "b"). Some of the records in the latter set (b, central to upper left) were re-scaled when the maximum pen displacement was shifted from 8 cm to 4 cm in order to stop damaging pens and motors during ongoing saturating oscillations. It is clear that the amplitude and duration of the 18-19 December 2000 events greatly exceeded those from 30 June 1997. Prior to mid-December, the 30 June 1997 events represented the largest amplitude tremor seen since 1994. Courtesy of CENAPRED.

Figure 32. Real-time seismic amplitude measurement (RSAM) plots for two Popocatépetl stations for the years 1997-8 and 2000, illustrating the extremely high seismic energy release seen in a time interval just over 7 days long (11-18 December 2000). This interval includes the acute increase seen on 18 December 2000. This ~7-day interval's energy release was eightfold larger than the total annual release in 1997. Courtesy of CENAPRED.

The episodes of quiescence and high-amplitude, low-frequency harmonic tremors occurred in such a pattern that they could be described as a load-and-discharge model, as suggested by the time-predictable model of Shimazaki and Nagata (1980). Using this paradigm, workers forecast the onset of the 18 December eruption and tremor episode.

Aerial photos taken on 16 December showed significant dome growth inside the crater (figure 33) and allowed correlation of the episodes of high amplitude, low-frequency harmonic tremor with periods of lava extrusion at very high rates. Analysis of the photos indicated that the dome grew at an average rate of ~180-200 m3/s during the episodes of intense harmonic tremor. This rate, which was not sustained, was about two orders of magnitude higher than any other previously observed.

Figure 33. A photograph taken looking into the crater at Popocatépetl, as viewed from the N on 16 December 2000. The substantial glacier on the N side lies covered by ash. Courtesy of CENAPRED.

At Popocatépetl, correlation spectrometer (COSPEC) measurements of SO2 flux have had yearly averages on the order of 5,000 metric tons/day (t/d). In contrast, during 13-19 December the estimates were in excess of 50,000 t/d. On 19 December the reported value was near 100,000 t/d.

Civil authorities were made aware of the high magnitude of the monitoring signals, the very high rate of lava production, and the growth of the largest dome yet observed. This motivated them to constitute, on 15-16 December, an emergency board. They declared a further increase in the alert level and defined a security radius of 13 km. This radius was suggested to include at least some of the most vulnerable towns, like Santiago Xalitzintla (centered ~15 km NE of the crater) and San Pedro Benito Juarez (with a few residences 10 km SE of the crater, but the main town at 12 km from it). Santiago Xalitzintla sits downstream of the E side of the largest glacier along one of the main N-flank drainages. San Pedro Benito Juarez lies on a fracture zone on the SE flank, an area where many of the largest tectono-volcanic earthquakes were located. Additionally, increased deformation was also detected using the geodetic network located on that fault. San Pedro Benito Juarez is an isolated town closest to a notch in the SE crater rim. This notch is believed to have formed by collapse on 24 February 1664 during an eruption similar to the current one.

Preventive evacuation of Santiago Xalitzintla, San Pedro Benito Juarez, and other towns began on late 15 December and early 16 December. The decisions regarding which other towns should be evacuated were made by authorities at the state and municipal level. This caused some towns, well outside the security radius of 13 km, to also be evacuated by decision of their mayors. About 41,000 people left the area. Around half left the region by their own will and means. The other half used resources provided by local civil protection authorities. Of these, ~14,000 accepted transportation to shelters where they remained for about 10 days. Others moved to stay with relatives or friends.

The total volume of fresh lava accumulated within the crater of Popocatépetl was estimated to be between 15 and 19 million cubic meters on 18 December, exceeding the combined volume of all the previous domes (figures 33 and 35). The estimated vertical growth rate of the dome was such that another 20 or 30 hours of tremor associated with the above-mentioned lava production rate could potentially have enabled the dome to begin escaping the confines of the crater. The rate slowed, however, and the dome's upper surface remained well within the crater (figures 33 and 35).

As anticipated by the applying the above-mentioned model, after a three-day period of relative quiescence, on the afternoon of 18 December, a new eruption began. The relatively low-explosivity, yet long-lasting eruption of 18-19 December (figure 34) ejected large amounts of hot debris on the flanks of the volcano in three episodes of incandescent fountaining. Ejected hot debris is believed to have ultimately flowed a maximum distance of 5-6 km from the crater. Some images of these eruptions were distributed by some news media, which had installed cameras around the volcano and broadcast images in real time. After 19 December activity decreased noticeably. The next expected period of unrest, suggested by the time-predictable model to ensue near 23 December (figure 35), did not occur, likely indicating that the rate of magma supply had changed. What was believed to be the first dome-destruction explosion of this episode occurred on 24 December, ejecting incandescent debris to a distance of 3.5 km from the volcano, and producing an ash plume estimated to reach 5 km above the crater. When the nature and size of this event was understood, authorities reduced the security radius to 12 km. No towns lie within that radius, and accordingly many people returned to their homes.

Figure 34. An ash-bearing eruption column rises from Popocatépetl on 19 December 2000, viewed from the N. This kind of activity was common during the energetic mid-December time interval and stimulated international attention (e.g. the media and websites of Reuters, Stromboli Online, and others). Courtesy of CENAPRED.

Figure 35. Aerial photograph taken looking into the crater at Popocatépetl, as seen from the NE on 23 December. Courtesy of CENAPRED.

Later explosive events failed to excavate substantial portions of the new dome. The current estimate as of 16 January 2001 was that ~10-20% of the new dome volume has been blown out by explosions recorded after 18 December 2000. In many of the previous dome growth-and-destruction episodes since 1996, most of the dome mass has been removed by small to moderate (VEI <= 2) explosions; a similar scenario may play out in the near future.

After several weeks of relative calm, significant activity resumed at Popocatépetl on 22 January. At 1458 a M 2.8 volcano-tectonic earthquake occurred on the E flank. This event was possibly a precursor to a large ash emission that started at 1615, and initially produced an ash plume several kilometers in height. Eight minutes later observers saw a more explosive phase throwing incandescent fragments around the crater. After several more minutes, pyroclastic flows were generated and moved 4-6 km down several ravines on the N flank. Ash emission from the crater was continuous and punctuated by intermittent explosions. By 1640, the ash plume towered more than 8 km above the summit crater. At 1800 fluctuating harmonic tremor, similar to that of December, was registered. At times the signals again reached saturation amplitudes; the tremor could have been associated with magma intrusion into the base of the crater, an idea also suggested to explain previous tremor events. Harmonic tremor lasted for ~30 minutes. Ashfall was documented in Santiago Xalitzintla, Atlixico, and parts of Puebla and Tetela del Volcán. At 2200 it was possible to see ejected incandescent fragments that fell up to 1 km from the crater. On 29 January (figure 36), pyroclastic flows caused some glacial melting. The pyroclastic flows initially reached up to 8 km from their source, halting in the drainage upstream of Santiago Xalitzintla. They triggered some glacial melting and in early February their deposits were remobilized and came to rest about 15 km from the crater, about 2 km upstream of Santiago Xalitzintla. As of 29 January Popocatépetl remained at a Stage 3 Yellow alert.

Figure 36. Popocatépetl on 29 January photographed looking S. The image captured the forceful ejection of an ash-laden cloud. Courtesy of CENAPRED.

Following an episode of intense volcanic activity at Popocatépetl during December 2000 and January 2001 (BGVN 25:12) volcanic activity through September 2001 consisted of periods of small-to-moderate emissions of steam, gas, and ash, several ash cloud-producing eruptions, periods of many high-frequency volcanic earthquakes, and fumarolic activity. In addition, a new lava dome grew within the crater left after a lava dome was destroyed in December 2000.

The Centro Nacionale de Prevencion de Desastres (CENAPRED) and the Washington Volcanic Ash Advisory Center (VAAC) noted several small-to-moderate sized eruptions during the report period. Large eruptions are discussed below, and others are in table 14.

Table 14. Eruptions at Popocatépetl during February-August 2001 not discussed in the report, based on information from CENAPRED, Washington VAAC, and the México City Meteorological Watch Office via the Washington VAAC. All heights are approximate values above the volcano.

Volcanic Activity during late January-February 2001. As of late January Popocatépetl was at Alert Level Yellow Phase Three, with a 12-km-radius restricted area. During the end of January through February several moderate-to-small eruptions occurred at Popocatépetl. On 30 January during 1530-1545 a moderate ash emission was visible on CENAPRED's video camera rising to ~1.5 km above the volcano's summit. The ~9-km-wide moderately-dense ash cloud extended from the summit to the N and NE. An eruption on 15 February at 1542 produced an ash cloud that rose to 2.5 km above the summit and drifted to the ENE. The intense phase of the eruption lasted about 15 minutes. The ash cloud was tracked using Geostationary Operational Environmental Satellite-8 (GOES-8) imagery as it drifted to the Gulf of México by 0102 the next day. The NOAA Operational Significant Event Imagery Support Team created a movie loop using images captured by GOES-8 that are available at http://www.osei.noaa.gov/.

New lava dome growth and destruction during March and April. Relatively low volcanic activity during the beginning of March consisted of small steam-and-ash emissions and periods of harmonic tremor. CENAPRED reported that beginning on 12 March volcanic activity rose to high levels, with harmonic tremor occurring for a cumulative hour and approximately 50 small emissions of steam, gas, and occasionally ash. An eruption at 2023 produced an ash column that rose 1 km above the summit and incandescent volcanic fragments were hurled up to 1 km away from the crater to the volcano's N flank.

On 13 March at 1953 another eruption produced an ash column that rose to 2 km. While flying over the volcano the same day CENAPRED personnel observed a new 100- to 150-m-diameter lava dome growing in the inner crater that was created after the December 2000 dome was destroyed. On both 14 and 15 March a cumulative hour-long period of harmonic tremor occurred and 55, and 73 emissions of steam, gas, and ash occurred, respectively. The lava dome was 200 m in diameter and about 40 m tall as of 15 March. On 16 March there was a larger number of volcanic emissions (95) than on the previous couple of days, but less harmonic tremor was registered (0.5 hour). Volcanic activity began to decrease on 17 March, with 38 emissions occurring and 15 minutes of harmonic tremor recorded.

During the remainder of March and early April volcanic activity related to the emplacement of the new lava dome occurred; there were episodes of harmonic tremor totaling up to 8 hours per day, a large amount of high-frequency tremor, an average of two tectono-volcanic earthquakes per day up to M 2.3, and fumarolic activity.

On 16 April at 1948 a moderate eruption produced an ash cloud that rose to 4 km above the volcano's summit and drifted to the SW (figure 37, a and b). The eruption also sent incandescent volcanic fragments up to 2 km from the crater to the volcano's NE and NW flanks. The 40-second-long eruption destroyed the lava dome that had formed within the crater over the course of the previous several weeks. After the eruption the level of volcanic activity stabilized, with a relatively low number of gas, steam, and ash emissions and episodes of harmonic tremor. On 17 April a small lahar traveled down the Achupashal Gorge.

Figure 37. For Popocatépetl, (a) a photograph showing the 16 April 2001eruption at 1949, and (b) thermal image of the 16 April eruption at an unstated time. In the thermal image, the ash cloud is visible rising to 4 km above the volcano's summit. Higher temperatures are represented by red and pink color shades in the area of fresh tephra deposition. The N flank of the volcano is shown. Hot material is visible on the upper NE and NW flanks of the volcano. Courtesy of CENAPRED.

Volcanic activity during late April-July. Following episodes of harmonic tremor during 28 April through early on 29 April a moderate eruption at 0819 produced an ash cloud that CENAPRED reported rose 2 km above the summit and quickly drifted to the ESE. A pilot reported that the ash cloud reached up to 3.5 km. The most intense phase of the eruption lasted approximately 1 minute. Extreme cloudiness obstructed clear views of the volcano, but scientist believe incandescent volcanic fragments were ejected during the eruption. Noise from the eruption was heard in San Pedro Benito Juárez (Puebla), 10 km SE of the volcano. By 0930 small amounts of ash fell in San Pedro Benito Juárez. Another small eruption occurred at 1310 and produced an ash cloud that rose 1.5-2 km above the volcano. After the eruptions volcanic activity returned to previous levels, with episodes of harmonic tremor and small volcanic emissions.

One of the many small eruptions during May occurred on the 13th at 2301 and ejected volcanic fragments up to 0.5 km away from the volcano's crater. Cloudy conditions prohibited observation of a possible accompanying ash cloud. The eruption was followed by an episode of harmonic tremor. A moderate-sized eruption on 31 May at 2136 sent incandescent material 2-3 km from the crater down the NE flank. The ash cloud produced from the eruption rose ~2 km above the volcano's summit and drifted to the W. The most intense phase of the eruption lasted approximately 1 minute. Harmonic tremor started about 90 seconds after the eruption began, and lasted about 5 hours. The following day a similar, but smaller, eruption at 0804 sent a steam-and-ash cloud to ~1.5 km.

Volcanic activity was relatively low in June, with small steam-and-ash emissions (table 4). CENAPRED reported that a moderate-sized eruption occurred on 3 July at 0410, which may have ejected incandescent volcanic fragments around the rim of the summit crater. Later that day, at 0648, a larger eruption produced an ash cloud that rose more than 4 km above the summit in a few minutes (figure 38). According to the Washington VAAC, at least three ash-producing eruptions occurred on 3 July; at 0425, 0648, and 0830. They reported that the 0425 eruption produced an ash cloud that was visible on GOES-8 imagery spreading in two directions at different heights; less than 1 km above the volcano one portion of the ash cloud drifted to the NW, and ~1-4 km above the summit it drifted to the SE (figure 39). Small amounts of ash fell NW of the volcano in the towns of San Pedro Nexapa, Amecameca, Tlalmanalco, San Rafael, Iztapaluc, and as far away as 35 km in Chalco.

Figure 38. Photograph of an eruption of Popocatépetl taken on 3 July 2001 at 0657. The northern side of the volcano is shown. Courtesy of CENAPRED.

Figure 39. Sketch showing the distributions of two portions of a Popocatépetl ash cloud in GOES-8 imagery on 3 July 2001at 0515. The enclosed hatched areas depict the location of volcanic ash. The portion of the ash cloud that drifted to the NW was ~ 1 km above the volcano and the portion that drifted to the SE, ~ 1-4 km above the volcano. Courtesy of Washington VAAC.

Based on information from pilot reports and ground observations, the Washington VAAC reported that the ash cloud was 9.3 km SE of México City airport (~65 km NE of the volcano) at 0930. Very light ash fell on runways at the Mexico City Airport, causing some airlines to briefly suspend takeoffs. CENAPRED's seismic data revealed that the explosive event lasted ~10 minutes, after which volcanism returned to low levels.

On 23 July CENAPRED reduced the Alert Level from Yellow Phase Three to Phase Two because volcanism was lower than it had been in December 2000 when the Alert Level was originally raised (BGVN 25:12). Under the new Alert Level, activity continued to be prohibited within a 12 km radius around the volcano, but controlled travel was permitted on the road between Santiago Xalitzintla (Puebla) ~10 km NE of the volcano and San Pedro Nexapa (State of México) ~12 km NW of the volcano, including Paso de Cortés.

New dome growth episode during August. A new episode of dome growth was first detected at Popocatépetl on 9 August when a significant increase in seismicity at the volcano lasted for about 24 hours. The seismicity was much lower than that detected in the interval beginning on 13 December 2000, a time when the highest amplitude tremor was recorded at Popocatépetl to date. A high-altitude flight took place on 10 August (sponsored by the Secretary of Communication and Transportation); it revealed that a new dome had been emplaced. It emerged at the bottom of the inner crater that formed after the December 2000 dome was destroyed (figures 40 and 41).

Figure 40. Sketch of Popocatépetl's summit crater and the new lava dome as they appeared on 10 August 2001. Courtesy of CENAPRED and Instituto de Geofísica, UNAM.

Figure 41. Photograph of Popocatépetl's new lava dome taken on 20 August 2001. Courtesy of CENAPRED and the Secretary of Communication and Transportation.

The lava dome's volume was estimated to be slightly more than 0.5 million cubic meters. Based on the assumption that the period of dome growth coincided with the period of maximum seismicity, the rate of growth was estimated to be 7-8 m3/s; less than 5% of the rates measured in December 2000. On 13 August the dome was 190 m in diameter and 30 m tall, about 5% the size of the December 2000 dome.

On 15 August at 1545 a new episode of high seismic activity began at the volcano. This episode was similar to the 9 August episode, but more steam-and-ash emissions with higher intensities occurred on 15 August. Seismicity further increased at 1800. The entire episode was attributed to a higher rate of lava extrusion. The waveforms and amplitudes of seismic signals were similar to those recorded on 13 December 2000; however, the total seismic energy release was about 30 % of the energy released on 13 December.

Small amounts of ash from the emissions fell NW and W of the volcano in San Pedro Nexapa, Amecameca, Ozumba, Atlautla, and San Juan Tehuiztitlán. Volcanic activity decreased on 16 August around 0115. During the night incandescence was seen at the summit and at 0538 incandescent fragments were ejected more than 500 m down the volcano's N flank.

After the August 15 increase in seismicity, seismic and volcanic activity returned to normal levels, with small volcanic emissions and periods of high-frequency and low-amplitude tremor. On 9 September during 0815-1605 an episode of frequent small- to moderate-sized eruptions began at Popocatépetl. The eruptions produced steam-and-ash emissions that rose to a maximum height of 1 km above the dome and drifted to the NW. During the night a small eruption sent incandescent fragments up to 200 m from the crater. Small amounts of ash fell in Ozumba (~15 km W of the volcano) and in Yecapixtla (~25 km SW of the volcano). Aerial photographs taken on 20 September revealed that the lava dome was visible within the crater.

During October 2001 through at least February 2002, volcanic activity at Popocatépetl consisted of small-to-moderate eruptions of steam, gas, and generally minor amounts of ash, along with episodes of harmonic tremor. Ash clouds rose up to ~6 km above the summit. The number of emissions ranged from 8 per week to 154 per day on 11 December. Because of the remote location and high elevation of the summit, the dome growth within the crater was often hard to constrain, although seismicity, remote sensing, and occasional flights over the summit did shed light on the situation. The following report is compiled from updates from the Centro Nacional de Prevencion de Desastres (CENAPRED) and from reports issued by the Washington Volcanic Ash Advisory Center (VAAC).

On 25 October an ash plume reached ~1.3 km above the summit and extended ~20-30 km N. An aerial photograph of the crater taken on 25 October showed that the lava dome, which had been reported on 20 September 2001, had subsided. In addition, a new 50-m-diameter lava dome had grown in the bottom of the internal crater. CENAPRED stated that the presence of the lava dome indicated the possibility of small explosions occurring within the following days to weeks. A similar situation was observed in April 2001.

On 10 November at 0324 a small explosion produced an ash column ~1 km high that drifted E. CENAPRED's volcano cam showed incandescent fragments being hurled up to 300 m to the E. During mid-November, seismometers recorded episodes with increased emissions and periods of harmonic tremor, activity that may be associated with lava dome growth. Similar events were observed on 15 August and 10 September.

During a flight over the volcano on 21 November a new 130-m-diameter lava dome was seen growing inside the internal crater (figure 42). The growth of this lava dome corresponded with increased emissions and seismicity on 17 and 18 November. Activity increased slightly on 25 November; three volcano-tectonic earthquakes occurred with magnitudes of 2.3, 2.4, and 2.7.

Figure 42. A sub-vertical aerial photo of Popocatepetl's concentric summit craters taken on 21 November 2001. The new (21 November 2001) dome lies within the innermost crater; it is 130 m in diameter, circular, and generally dark in color; much of the rest of the summit is gray to white due to varying degrees of ash on top of snow and ice. N is towards the top; the high point on the crater rim is at the bottom. Courtesy CENAPRED.

On 6 December at 0615, two M-2.3 volcano-tectonic earthquakes occurred, located 3 km and 5 km below the crater. Fumarolic activity remained low.

A flight on 10 December confirmed the growth of a new dome, with an estimated diameter of 190 m. During 1830 to about 2200, several small-to-moderate emissions ejected incandescent fragments up to ~1 km away. This increase in activity was taken to herald the destruction of the 21 November dome. At 1958, ash was visible over the volcano on satellite imagery, and at 0732 on 11 December a bright hotspot was visible in shortwave imagery.

On 17 December at 2212 a small explosion ejected incandescent fragments up to 500 m to the E and produced an ~1-km ash column that drifted E. An ~2-hour episode of low-amplitude harmonic tremor accompanied the event.

Several moderate short explosions produced ash-and-gas columns on 18 December. An explosion at 1124 sent incandescent fragments 1 km to the ENE flank and produced an ash column to ~2.5 km above the summit that drifted NNE. Episodes of low-amplitude harmonic tremor accompanied the activity. At 1215 an ~18-m-wide plume was visible on satellite imagery reaching ~2.4 km above the summit and extending 50 km from the volcano.

A short episode of low-amplitude harmonic tremor on 19 December was followed by an explosion at 1926 that sent incandescent fragments 2 km to the E and NE. An associated seismic event lasted ~3 minutes, with a more intense phase occurring for 2 minutes, after which the seismicity returned to stable levels. Another low-amplitude harmonic tremor episode occurred the following day.

On 22 December at 1735 an explosion emitted an ash column to 2.5 km that drifted NE. The event was accompanied by an episode of low-amplitude tremor that lasted for 4 hours. Following the event an ash cloud was visible on satellite imagery reaching up to ~5.6 km above the summit. The ash cloud was centered 54 km ESE of the summit and was up to 15 km wide. One light ash fall was registered in some parts of the city of Puebla, ~60 km E of the volcano. Following the 22 December explosion, activity decreased to a low level, but episodes of low-amplitude harmonic tremor continued.

On 7 January an M 2.6 volcano-tectonic earthquake was located 3.3 km below the summit to its SE. Emissions produced small amounts of ash that rose to 800 m, and a gas plume rose 500 m and moved SE. CENAPRED suggested the plume was related to the cooling of the December 2001 lava dome. A photograph taken on 11 January showed that this dome had become partially destroyed. On 11 and 13 January, emissions sent plumes to ~2.0 km above the summit. The 11 January plume was 3 km wide and was visible on satellite imagery extending 14 km from the volcano.

At 0517 on 23 January a moderate eruption was accompanied by continuous tremor. During the eruption incandescent volcanic fragments were ejected short distances from the crater, and a steam-and-ash cloud was produced. According to pilot reports, the cloud rose to ~3.6 km above the summit. Satellite imagery showed the cloud drifting to the NE (figure 43), and by 1845 on 23 January it was visible in the vicinity of the city of Poza Rica, ~250 km to the NE. The cloud continued drifting NE over the Gulf of Mexico until it was no longer visible on satellite imagery. Small amounts of ash fell at Paso de Cortés (~7.5 km N of the summit) and the town of Tlaxcala (50 km NE).

Figure 43. Satellite imagery related to an eruption at Popocatépetl on 23 January 2002 (capturing the scene at 1316 UTC). The image shows a distinct white ash plume extending NE ~ 40 km from the volcano. Arrow points approximately towards the summit (the source of the plume); distance from the summit to Puebla is ~ 60 km; N is up. Courtesy NOAA.

The 23 January eruption marked the formation of a new lava dome, which was seen on an aerial photograph taken on 24 January. At that point it was 180 m in diameter and 150 m high. Thus, its volume measured ~20 times smaller than the dome formed in December 2000 (BGVN 25:12). On 28 January, seven volcano-tectonic earthquakes occurred with magnitudes of 1.8 to 2.3. A small steam plume drifted NW.

After several weeks of low-level activity, seismicity increased slightly at Popocatépetl during 15-16 February 2002. Harmonic tremor and low-magnitude volcano-tectonic earthquakes were recorded. CENAPRED stated that the activity was possibly related to the ascent of magma and to the formation of yet another new dome.

On 23 February at 2156 a minor explosion ejected incandescent fragments up to 200 m before they fell back into the crater, and a low-volume ash plume reached 700 m above the summit. The next day, short isolated harmonic tremor episodes accompanied several small emissions that produced gas plumes. An episode of low-amplitude harmonic tremor on 28 February lasted for ~6 hours.

Heightened activity, including tremor, continued through at least 28 February. CENAPRED suggested that this activity could be related to growth of another small dome after the destruction of the 21 November dome, but that remained unverified. CENAPRED reported that the activity during the end of February was similar to that during August, September, November, and December 2001, and during January 2002. The volcano was last reported at Alert Level Yellow Phase II during mid-February.

During March through at least late June 2002, volcanic activity at Popocatépetl consisted of small-to-moderate, but at times explosive, eruptions of steam, gas, and generally minor amounts of ash, along with episodes of harmonic tremor. Ash clouds rose up to ~2 km above the summit. Because of the remote location and high elevation of the summit, the dome growth within the crater was often hard to constrain, although seismicity and occasional flights over the summit did shed light on the situation. The following report is compiled from updates from the Centro Nacional de Prevencion de Desastres (CENAPRED) and from reports issued by the Washington Volcanic Ash Advisory Center (VAAC).

March began with activity at low and steady levels with up to 18 small steam-and-gas emissions per day and occasional episodes of harmonic tremor. The amount of ash emitted was generally minor. Occasional M 3 volcano-tectonic (VT) events were recorded. Low fumarolic activity began on 4 March and was frequently visible throughout the report period. Overflight observations on 7 March confirmed the presence of a lava dome in the crater (figure 44). A gas-and-steam plume reached ~2 km above the crater on 9 March. According to CENAPRED, the activity implied the possibility of low-level explosive activity in the coming days or weeks.

Figure 44. Aerial view from the NE on 7 March 2002 of the crater of Popocatépetl. The darkest circle in the center of the crater represents the newest lava-dome growth. Courtesy CENAPRED.

Activity increased during 26-27 March when 42 gas and steam emissions reached 200-500 m above the crater, accompanied by small amounts of ash and low-amplitude harmonic tremor. The Washington VAAC issued a volcanic ash warning based upon seismic observations that indicated a possible ash-bearing eruption, but no ash was visible in satellite images. Activity decreased to levels similar to earlier in the month and continued at those levels through early April.

At 0438 on 8 April, observers recorded a moderate eruption with explosive characteristics accompanied by some visible incandescence. An accompanying ash cloud moved E towards the coastline and diffused within 24 hours. After a M 2.3 VT earthquake was recorded at 0545 on 8 April, activity returned to steady levels.

Activity remained low through mid-April, with the exception of a brief period around 11 April when observers detected a slight increase in low-amplitude tremor and fumarolic activity. An increased number of small-to-moderate exhalations per day (up to 52) accompanied by episodes of low-level harmonic and high-frequency tremor, and weak VT earthquakes characterized increased activity that began in late April and lasted through early May. According to CENAPRED, this activity was most likely related to motion of small amounts of magma towards the surface and growth of the lava dome within the crater.

An air photo taken on 29 April (figure 45) by the Department of Federal Roads showed a small dome ~170 m in diameter. On 1 May CENAPRED reported an ash plume moving W at 1.0 km above the summit. No ash was visible on satellite imagery.

Figure 45. Air photo of the Popocatépetl crater taken by the Department of Federal Roads on 29 April 2002. The darkest circle in the left-center of the photo is the newest lava dome, measuring 170 m across. Subsequent flights indicated that explosive activity on 12 May destroyed part of this dome. Courtesy CENAPRED.

Activity increased slightly during mid-May with 33 small-to-moderate exhalations and 1 hour of low-amplitude tremor on 10 May. At 0609 on 12 May, a small explosive eruption occurred, ejecting incandescent fragments on the N flank up to 500 m from the crater. During the next few days, CENAPRED reported increased numbers of exhalations per day (up to 124 on 14 May) of steam, gas, and sometimes small amounts of ash. It was later determined from overflight observations that this explosive activity destroyed part of the growing dome.

This period of increased activity decreased beginning around 17 May. During the rest of May, activity was again characterized by numerous (up to 66) small-to-moderate gas-and-steam exhalations accompanied by small amounts of ash and periods of harmonic tremor. Fumarolic activity continued at the surface. A pilot reported an ash cloud in the region on 21 May.

Activity declined to steady, low levels through June with the average number of exhalations per day dropping to less than 10, occasional isolated harmonic tremor episodes of ~15 minutes duration, and as many as five VT earthquakes per day (M 2.5).

On 17 June at 1136 an ash plume extended up to 2 km above the summit and drifted to the WSW. Shortly thereafter, CENAPRED recorded high-frequency tremor for almost 8 hours and four VT events (M 2.0-2.2). The resulting ash cloud moved across Mexico to the SW. During the following days the volcano quieted but continued to emit gas, steam, and ash in small quantities with episodes of harmonic tremor lasting less than an hour. On 27 and 29 June ash plumes reaching up to 2 km above the summit were accompanied by periods of harmonic tremor lasting up to 2 hours. The Alert Level remained at Yellow throughout the report period.

During July-October 2002, volcanic activity at Popocatépetl consisted of small-to-moderate, but at times explosive, eruptions of steam, gas, and generally minor amounts of ash. Explosions on 1 and 2 July produced ash plumes that reached 2 km and 700 m above the crater, respectively. Volcano-tectonic (VT) earthquakes (M 1.8-2.9) occurred almost daily. The earthquakes were located mostly to the S and E at depths up to 8 km beneath the crater. Isolated episodes of low-amplitude harmonic tremor were registered, typically for a few hours daily.

The Centro Nacional de Prevencion de Desastres (CENAPRED) reported that during most of July through mid-August, up to 25 small-to-moderate emissions per day were accompanied by steam, gas, and sometimes small amounts of ash. The number of exhalations per day increased during 22-24 July (43, 80, and 55) and 15-17 August (68, 58, and 70). Around 25-45 exhalations occurred per day through the end of August. During September and October, no more than 26 exhalations were registered per day.

Activity reported by CENAPRED in July was probably related to changes in morphology of the intracrater dome (BGVN 27:02 and 27:06). Compared to an aerial photo taken on 29 April (figure 46), an image on 22 May 2002 (figure 47) showed that the dome had diminished in size.

Figure 46. Vertical aerial photo of Popocatépetl taken on 29 April 2002. The top of the image is generally towards the N. Courtesy CENAPRED.

Figure 47. Vertical aerial photo of Popocatépetl taken on 22 May 2002. The photo provided evidence that the dome was diminished in size compared to 29 April 2002 (figure 46). The top of the image is generally towards the NNW. Courtesy CENAPRED.

CENAPRED stated that future activity could consist of isolated minor explosions with emission of incandescent fragments out to short distances from the crater or emissions of variable quantities of ash. The Alert Level remained at 2, and CENAPRED recommended that people avoid the zone extending out to 12 km from the crater, although the road between Santiago Xalitzintla (Puebla) and San Pedro Nexapa (Mexico State), including Paso de Cortés, remained open for controlled traffic.

From November 2002 through mid-February 2003, volcanic activity at Popocatépetl was similar to that during July-October 2002 (BGVN 27:10). Activity consisted principally of small-to-moderate eruptions of steam, gas, and minor amounts of ash, and occasional explosions that ejected incandescent fragments for short distances. Larger explosions on 6 November, 18 and 23 December 2003, 9 January, and during 4-10 February 2003 produced ash plumes that reached approximate heights of 4, 2, 2, 3, and 2 km above the crater, respectively. Volcano-tectonic (VT) earthquakes (M 2.0-3.2) occurred frequently, most located to the SE, N, and E at depths up to 7.5 km beneath the crater. Episodes of harmonic and low-amplitude tremor were registered almost daily, typically for a few hours.

Until November, the daily emissions reported by the Centro Nacional de Prevencion de Desastres (CENAPRED) typically numbered from as few as 5 to as many as 20. In late November, this number increased markedly with 78 detected on 24 November and 40 the following day. Subsequently the daily number of these small-to-moderate emissions occasionally exceeded 30 through mid-February 2003.

New episodes of low-frequency tremor, beginning on 19 November, signaled the growth of a new lava dome within the crater. Aerial photographs obtained by the Mexican Ministry of Communications and Transportation on 2 December confirmed the presence of a fresh lava dome with a base diameter of 180 m, and a height of ~52 m. CENAPRED reported that the explosive activity reported on 18 and 23 December was related to the destruction of the lava dome. Photographs of the lava dome taken on 9 January revealed that the dome's inner crater had subsided. The volume of dome material ejected during the December explosions was calculated to be ~500,000 m3.

CENAPRED stated that explosive activity beginning in mid-January was related to the growth of a new lava dome in the crater. On 22 January a significant increase in volcanic microseismicity was recorded. According to the Washington Volcano Ash Advisory Center, on 25 January an ash emission reached ~10.7 km altitude. The explosion on 4 February ejected incandescent volcanic material that fell as far as ~2 km down the volcano's flanks. Similar emissions continued and were related to partial destruction of the lava dome. According to CENAPRED, as long as there are remains of a lava dome in the crater, a significant chance of further explosive activity remains, including ash emissions and incandescent ejections around the crater. The Alert Level remained at Yellow (second on a scale of three colors) and CENAPRED recommended that people avoid entering the restricted zone that extends 12 km from the crater. However, the road between Santiago Xalitzintla (Puebla) and San Pedro Nexapa (Mexico State), including Paso de Cortés, remained open for controlled traffic.

Volcanic activity at Popocatépetl during March-July 2003, as reported by the Centro Nacional de Prevención de Desastres (CENAPRED), was similar to that from July 2002 to February 2003 (BGVN 27:10 and 28:02). Activity was comprised principally of multiple exhalations (some with significant ash), volcano-tectonic (VT) earthquakes, and explosions. Daily exhalations averaged 10-30 during March, 5-25 during April and May, and _50 during July. VT earthquakes in these months were M 2-3 at depths of 2-5 km located E or SE of the volcano.

On 28 April and 10 May, low-frequency harmonic tremors during the VT events attained moderate, but significant, amplitude levels lasting 13 and 4 hours, respectively. Eruptive activity during June, presumed to be predominantly phreatic, increased and caused ash-bearing exhalations and explosions. Another significant tremor episode was detected on 8 June. Eruptions on 20 and 28 June caused minor ashfall on some towns near the volcano. During July, many exhalations were explosive and carried significant ash. The largest explosive events in July were recorded on 1, 15, 19, and 25. The event of 19 July (figure 48) caused light ashfall as far as the southern metropolitan area of Mexico City. Aerial photography of the crater on 30 April and 19 May indicated no evidence of new lava dome emplacement during the report period.

Figure 48. Photograph of an eruption at Popocatépetl volcano, 19 July 2003. Courtesy of CENAPRED.

The Centro Nacional de Prevención de Desastres (CENAPRED) provided daily reports for Popocatépetl describing the comparatively quiet interval of 1 August-5 December 2003. When the volcano was visible it typically gave off minor gas plumes characterized by statements such as "low fumarolic activity" and "without important emissions." The hazard status remained at Yellow-Phase II.

A series of aerial photos enabled scientists to view the state of the crater floor on 21 July, 25 August, 17 October, and 6 November 2003. All of these failed to disclose the growth of an external lava dome. In addition, some of the reports suggested that the floor of the inner crater had subsided.

On the vast majority of days during the reporting interval there were fewer than 10 exhalations, and on ~45% of these days, four or fewer exhalations. Although some resulting plumes contained ash, the vast majority of exhalations (which are detected seismically) were described as low intensity. In a few cases, particularly in August and on 1 September, exhalations occurred 20-89 times per day and reached moderate intensity. Daily reports on some of those days cited elevated groundwater levels due to recent snow or rainfall (rather than deeper magmatic processes) as the cause of increased exhalations.

The most exhalations were registered during August 2003, a month when six days had 12 or more exhalations. In contrast, during September-November 2003 there were only four days reported to have had more than 10 exhalations. Exhalations exceeded twenty on 2 August (35), 23 August (60), 28 August (89), and 1 September (43). On days when exhalations exceeded twenty, often (though not always) one or more of the plumes contained small amounts of ash. For example, an ash-bearing plume was noted at 0300 on 2 August. On that day low-amplitude tremor registered for about 1 hour. The 60 exhalations on 23 August were described as small to moderate, generating plumes composed of gas and steam. They were thought to be related to intense rains during the preceding days. The 89 exhalations on 28 August 2003 were similarly described as low to moderate and accompanied by small steam and gas emissions. On 28 August at 1330 an eruption occurred that bore a low density of ash. The plume reached a height of about 1,500 m above the crater; it dispersed towards the W with no reported ashfall. This event was accompanied by episodes of high-frequency and low-amplitude tremor.

Tremor frequently went unreported. When mentioned, CENAPRED said it took place for up to approximately 2 hours per day, but in some cases only several minutes per day. Small (M ~2-3) earthquakes were repeatedly noted during the interval, including a few in the last half of August, several in September, two in October, and seven in November. During 1-5 December one such earthquake occurred. During the August-5 December interval the largest earthquake, M 2.9, took place on 5 November 2003.

Several examples can serve to illustrate the reported data on many of these earthquakes, which occurred in vicinity of the volcano at depths of a few kilometers. On 20 August seismometers recorded an M 2 volcano-tectonic earthquake 1 km N of the summit at 4.8 km depth. At 2312 on 7 September there was a M 2.2 volcano-tectonic earthquake 6.5 km SE of the crater. At 2137 on 8 September a M 2.3 volcano-tectonic earthquake 5 km below the crater registered.

During 2004, Popocatépetl showed an overall low level of activity. Apart from a few low-intensity exhalations, no significant seismicity, deformation, or geochemical changes in spring waters were detected. The crater (figure 49) did not show significant morphological changes other than hydrologic effects, and no evidence of lava dome emplacements were observed. During December, relatively low-level volcanism prevailed, including low-intensity steam-and-gas emissions (table 15). An aerial photograph taken on 10 December showed subsidence in the inner crater and no external lava dome at the bottom of the crater. The Alert Level remained at Yellow Phase II.

Figure 49. Annotated aerial photograph of Popocatepétl's summit area taken 12 November 2004. Courtesy of CENAPRED; the Mexican Secretary of Communications and Transportation; and Servando De la Cruz, UNAM.

Table 15. Summary of various observations at Popocatépetl during December 2004 (chiefly visual confirmations of ongoing emission). Courtesy of CENAPRED.

At the end of December 2004, however, a slight increase in seismic activity was detected (table 15). On 20 and 29 December 2004 two exhalations, small yet exceeding the average for the year, were followed in early January 2005 by a series of phreatic explosions. The major events were detected on 9 January at 2245 and on 22 January at 2358. These were the largest events detected in the past 15 months. In both cases, light ashfall was reported on the towns of Cuautla (< 40 km SSW of the volcano), and San Martín Texmelucan (37 km NE of the crater). In an aerial photograph taken on 14 January 2005 the inner crater appears deeper than previously shown, with no evidence of magmatic activity (figure 50).

Figure 50. Annotated photograph of Popocatépetl's summit area taken 14 January 2005. A label identifies a small craterlet on the southern inner wall of the inner crater. These type of craterlets have been repeatedly formed by moderate explosions or exhalations. They have tended to be ephemeral, lasting only until the next event. Courtesy of CENAPRED; the Mexican Secretary of Communications and Transportation; and Servando De la Cruz, UNAM.

The last report on Popocatépetl covered the period December 2004-January 2005 (BGVN 30:01). This report covers the rest of 2005 (February-December) and comes from the Mexican group Centro Nacional de Prevención de Desastres (CENAPRED).

The volcano has been relatively quiet with daily, low-intensity exhalations of steam and gas sometimes containing minor ash. Some highlights of the reporting interval follow (both in text and in tables), essentially a list of ash-bearing eruptions that usually rose 1-3 km above the crater rim. On 1 December an ash plume rose 5 km.

At 0723 on 13 July an exhalation of moderate intensity produced an ash emission that reached 2 km above the level of the crater. Reports were received of ash fall in Yecapixtla and Ocuituco in the state of Morelos, respectively ~ 30 and ~ 23 km SW of the volcano.

At 0906 on 21 July an exhalation of moderate intensity followed a tremor of high frequency and produced an ash emission that reached 2 km.

At 0313 on 29 July, a moderate exhalation with an explosive component occurred, throwing incandescent fragments onto the E side of the cone, out to a distance of about 1 km. The duration of the intense phase was ~ 1 minute; it was preceded by high-frequency tremor, the same signal was observed for 15 minutes after the explosion. The height of the steam, gas, and ash plume was estimated at 2 km. There were no reports mentioning ash fall.

At 0614 on 29 July a new exhalation with explosive component occurred producing an ash column ~ 2.5 km above the crater. The winds directed the plume NW. The duration of the intense phase was 30 seconds followed by high-frequency tremor (4 minutes). Reports of ashfall in the Milpa Alta area (SE of Mexico City) were received.

At 0819 on 30 July high-frequency tremor started, associated with an ash column. The plume reached 2 km and was directed NW. The emission continued in its most intense phase until 0845. On 2 August at 1247 an exhalation of moderate intensity produced an ash emission that reached 2 km.

At 0305 on 24 October an exhalation of moderate intensity occurred. It was followed by a tremor of high-frequency. The resulting ash plume reached 1.2 km and traveled NE.

At 0653 on 1 December a moderate exhalation sent an ash column to a height of 5 km above the summit, the highest reported ash column in 2005. It was dispersed towards the ENE. The intense episode lasted 2 minutes and was followed by high-frequency tremor lasting 30 minutes. A small ash fall was reported in Amecameca and a warning was issued noting that minor ash fall could occur within the next few hours in Tlaxcala and Puebla states. At 0920 another, smaller eruption produced an ash column 2.5 km above the crater, which also blew ENE.

An eruption that occurred at 1653 on 4 December resulted in reports of ash fall in the states of Tlaxcala and Puebla.

At 2225 on 13 December an explosion at Popocatépetl ejected incandescent fragments over its SE flanks. The explosion lasted ~ 1 minute and was followed by 30 minutes of tremor. The explosion produced a 2.5 km ash column carried by winds to the SE. After the explosion, the volcano returned to its previous low level of activity.

At 0457 on 18 December a small explosion was detected at the Popocatépetl. This event produced an ash column that reached 2.5 km over the summit. The plume traveled towards the NE. The explosion lasted ~ 1 minute and was followed by 8 minutes of high-frequency tremor.

At 0447 on 25 December a moderate exhalation with a minor explosive component occurred. Some incandescent fragments fell on the E side of the cone extending 700 m from the crater edge. The accompanying ash column reached a height of about 3 km moving ENE. The total duration of seismicity associated with this event was nearly 3 minutes, of which 50 seconds corresponded to the intense phase. There were no reports of resulting ash fall.

All of the reported ash-producing events for the February-December 2005 reporting interval are listed in table 16. Table 17 contains the reported earthquakes.

During first half of 2006, several ash plumes rose to ~ 7-8 km altitude

The last report on Popocatépetl covered February-December 2005 (BGVN 30:12). This report covers January-June 2006. Throughout this reporting interval, the warning level remained at Yellow. Seismicity is summarized on table 18.

Table 18. Recorded earthquakes near Popocatépetl during April-June 2006. Courtesy of CENAPRED.

On 6 January 2006, a small explosion occurred at Popocatépetl around 0042. According to the Washington VAAC, the resultant ash plume was visible on satellite imagery and its top reached ~ 5.8 km altitude, extending NE. Centro Nacional de Prevención de Desastres (CENAPRED) reported that after the explosion overall activity decreased to previous levels. During 24-30 January, several emissions of gas, steam, and small amounts of ash occurred. A moderate explosion on 26 January at 0957 produced an ash plume that rose to ~ 8.4 km altitude and drifted NE.

Throughout the month of February, several small-to-moderate emissions of steam, gas, and ash occurred. On the 4th, an explosion produced a plume that rose to ~ 6.7 km altitude. Aerial photos taken on 10 February showed a 130-m-diameter lava dome at the bottom of the crater. At 0528 on 24 February an M 2.3 earthquake was detected and was located 0.5 km to the N of the crater at a depth of 4.1 km.

During April-June, the volcano issued several small emissions of steam, gas, and ash; reports also noted several small coincident earthquakes. At 1807 on 23 May, an ash emission was observed that reached a height of ~ 7.4 km altitude. The ash column was dispersed towards the SE and was followed by a high-frequency, low-amplitude tremor signal that lasted 90 minutes and then returned to previous low levels.

Centro Nacional de Prevencion de Desastres (CENAPRED) reported only sporadic, modest activity at Popocatépetl during early 2006 through April 2007. Based on information from the Mexico City Meteorological Watch Office (MWO), and the Washington Volcanic Ash Advisory Center (VAAC), there were five occasions when ash plumes rose substantially. On 25 and 27 July 2006 ash plumes rose to an altitude of ~ 9.8 km. On 18 and 20 December 2006, ash plumes rose to an altitude of ~ 6.7 km and 7.9 km, respectively. In April 2007, ash plumes rose to ~ 7.6 km on the 1st, and to ~ 7.3 km on the 3rd.

In August 2006, the lava dome that had been irregularly growing since July 2005 covered the floor of the internal crater and began a piston-like growth on the top of the previous dome. The enlarged dome can be seen in an aerial photography taken in 24 November 2006 (figure 51). This formation of the dome was the twenty-sixth such event since 1996.

Figure 51. Aerial photo taken 24 November 2006 showing the growing lava dome at Popocatépetl.The dashed white line defines the dome edge. The lava dome that started growing in July 2005 has covered the floor of the internal crater and began growing on the top of the previous dome. The white areas outside the inner-crater rim are snow cover. Courtesy of the government of the State of Puebla, Mexico.

On 4-5 August and 1-3 November 2006 episodes of large-amplitude harmonic tremor (figure 52) were believed to reflect an increased rate of dome growth. The accumulated volume of the lava dome between November of 2005 and November of 2006 was estimated to be 1,299,000 m3. The average rate growth over that interval is around 0.04 m?/s. Assuming that the dome grows only during the tremor episodes, the rate would be ~ 6.75 m3/s.

Figure 52. Evidence of a large-amplitude, multiband harmonic tremor, showing clear frequency peaks in its spectrum detected in August 2006 at Popocatépetl. The combination of the frequencies appear as moiré shadows in the paper recording.Courtesy of CENAPRED.

Incandescence at the summit was recorded by the CENAPRED camera on 3 August and 4-5 September 2006. Over 27-29 October 2006, eigth small explosions ejected incandescent debris on the slopes surrounding the crater. During November and December 2006, more episodes of low amplitude tremors were recorded. From August to December 2006, 77 volcano-tectonic micro-earthquakes were detected, with magnitudes ranging between 2.0 and 3.0. From these, 66 were located below the crater at depths ranging between 3 and 7 km (figure 53).

Figure 53. Location and depth of micro-earthquakes on Popocatépetl recorded during August to December 2006. Courtesy of CENAPRED.

Hot spots at the summit were detected on satellite imagery by the Washington Volcanic Ash Advisory Center (VAAC) on 7-8 January 2007. According to the Washington VAAC, a puff with little ash content emitted from Popocatépetl was reported from the MWO and visible from the camera operated by CENEPRED on 14 February 2007. A very diffuse plume was seen drifting to the E on satellite imagery. Base on an aerial photograph taken on 24 January 2007, CENEPRED reported that the lava-dome dimensions have slightly increased since 24 November 2006.

Our most recent report on Popocatépetl (BGVN 32:04) described minor explosions, sporadic ash plumes, and lava dome growth during 2006 through April 2007. The current report discusses activity from April 2007 through April 2009, when many small ash plumes were noted.

From April 2007 through April 2009, the Centro Nacional de Prevencion de Desastres (CENAPRED) reported modest activity at Popocatépetl, consisting largely of numerous low intensity earthquakes and tremors (figures 54 and 55), and constant degassing of low intensity steam and gas, often accompanied by ash emissions of variable intensity. Based on information from the Mexico City Meteorological Watch Office (MWO), and the Washington Volcanic Ash Advisory Center (VAAC), there were 17 occasions when ash plumes rose at least 1 km above Popocatépetl's 5.4 km summit (table 19).

Figure 54. Popocatépetl upper flanks seen looking SSE in October 2008. During the reporting interval, steam plumes often hung over Popocatépetl's summit. The summit area is steep and glacial covered. The volcano's crater is deep and contains a growing dome of uncertain volume. Photo taken by Julie Roberge, UNAM.

Figure 55. Histogram of selected annual activity at Popocatépetl. The number of earthquakes, tremors, gas, and gas + ash episodes per month between January 2007 and 1 April 2009. The amount of ash in the eruptions was modest, compared to total gas emissions. Courtesy of Julie Roberge.

Table 19. Tabulation of ash plumes rising at least 1 km above Popocatépetl's summit between 1 April 2007 and 1 April 2009. Data provided by the Mexico City Meteorological Watch Office (MWO), and the Washington Volcanic Ash Advisory Center (VAAC) and a web camera operated by the Centro Nacional de Prevención de Desastres (CENAPRED).

According to information provided by the Mexican National University geologist Julie Roberge, the tremors lasted from minutes to hours and varied in frequency and amplitude, but were mostly of low amplitude. The microearthquakes were also of low magnitude (M 2-3) with variable depths; epicenters were typically within 10 km of the crater. Plumes consisting of gas, and gas and ash, and seismicity consisting of earthquakes and tremor varied during 2007 through April 2009 (figure 55).

According to Roberge and others (2009), the deep degassing observed in the ongoing eruption of Popocatépetl may indicate an essentially intrusive event, rather than a convective process. The hypothesis of deep magma degassing beneath Popocatépetl is consistent with observations regarding degassing at the summit that suggest separation of magma and gas at depth beneath the volcano. According to additional information provided us by Roberge, the high gas flux is not associated with processes in the central conduit. The volcano has an elliptical crater (600 m by 800 m). Most of the lava that formed domes was extruded through the crater's major central vent (about 30 m wide). However, several other vents were formed in the crater during the explosive events of 1995. These vents are aligned N-S, and the largest has been the site of long term degassing but only rare extrusion of lava. The smaller secondary vents are ephemeral and their activities depend on the explosive events that reopen them. Often, the most vigorous release of gas occurs from the E vent, and thus much of the degassing seems unrelated to the central vent and conduit from which the lava domes form.

Between April 2007 and April 2009, thermal anomalies at Popocatépetl were detected every month by MODVOLC. The number of thermal anomalies per month ranged from three to seventeen, mostly one pixel, but occasionally two pixels, and once three pixels.

As reported in BGVN 32:04, a lava dome irregularly growing since July 2005 covered the floor of the internal crater. People studying the volcano have lacked an image of the dome and crater since 2007, leaving its later status and volume uncertain.

This report discusses the time interval April 2009 through mid-October 2010, a period in which activity at Popocátepetl continued in the form of tremor, numerous earthquakes, and many small ash plumes. Our most recent report on Popocatépetl volcano (BGVN 34:03) described activity from April 2007 through April 2009.

According to a report from Julie Roberge, Universidad Nacional Autonóma de México (UNAM), summarizing activity at Popocatépetl in 2009, the Centro Nacional de Prevencion de Desastres (CENAPRED) reported no significant changes for that year in the volcanic activity, which consisted of low-intensity earthquakes and tremors and constant degassing (exhalation), sometimes accompanied by ash emissions of variable intensity (figure 56). Table 20 gives a listing of plume activity from 1 April 2009 through 4 October 2010 for plumes observed to have some ash. Based on information from Roberge, CENAPRED, the Mexico City Meteorological Watch Office, and the Washington Volcanic Ash Advisory Center, there were ~11 ash plumes reported that rose ~1 km above the summit. Other plumes were seen during this period, many of them emitting steam and gas, sometimes with slight amounts of ash, but whose altitudes were unreported.

Figure 56. Photo of Popocatépetl showing an ash emission on 9 October 2009. The gas and ash plume column reached up to 3 km above the summit. Courtesy of CENAPRED.

Roberge reported that, during 2009, the number of gas exhalations decreased from January through April, returning to highs in June and August, and decreasing again into December. As was observed in previous years, the amount of ash in the eruptions was modest compared to total gas emissions (figure 57). Other than peaks in January and August, the number of earthquakes in 2009 stayed low throughout the year; the number tremor episodes seemed to be inversely related to gas emissions (figure 58).

Figure 57. Histograms of the number of gas exhalations and percentage of exhalations containing ash in 2009. Courtesy of Roberge (2010).

Figure 58. Histogram of the number of earthquakes and tremors in 2009. Courtesy of Roberge (2010).

MODVOLC Thermal Alerts. Between 5 November 2009 and 20 October 2010, thermal anomalies at Popocatépetl were reported as being detected every month by the MODVOLC system. The number of days showing thermal alerts per month ranged from one to fifteen (the latter, March 2010), mostly one pixel imaged during an overflight, but occasionally two pixels. Unusually, from 4 May to 5 November 2009, no thermal alerts were detected; one alert was measured in May 2009, and 4 in April 2009.

Popocatépetl continued to be active during September 2010 to 13 December 2011 with explosions, tremor, and frequent gas-and-steam emissions occasionally containing ash (figure 59). This report continues the table in the previous report (BGVN 35:08) that tallies the seismic activity and ash emissions (table 21). Figure 60 shows the proximity of the volcano to population centers.

Figure 59. Photograph of the lava dome at Popocatépetl taken on 8 September 2011. Courtesy of CENAPRED.

Figure 60. Map of Popocatépetl in relation to Mexico City and other large communities. Circular areas are approximate; they show some hazard zones, including the innermost "Red" zone, which is within 5 km of the summit and excludes the public from entry. Courtesy of CENAPRED.

During the reporting interval, there were a large number of MODVOLC thermal alerts for Popocatépetl.

Arana-Salinas and others (2010) discuss the Ochre Pumice Sequence, a major (VEI 6) event that occurred ~ 5,000 years ago. That unit contained a sequence of pyroclastic flow and fall deposits that covered ~ 300 km2 directed NNE. The erupted magma amounts to a (dense rock equivalent) volume of ~ 2 km3. The authors stated that, depending on the wind direction, an equivalent event today would impact 15 million residents of Mexico City (the Capital), Puebla, Atlixco, and Cuautla and elsewhere, and it would severely damage infrastructure.

Small but frequent ash and explosive events have continued from Popocatépetl since our last report in November 2011 (BGVN 36:11). Here we tabulate and briefly discuss the numerous explosions from 14 December 2011 to 26 June 2012, highlighting the events that led authorities to elevate the Alert Level to Yellow Phase 3 on 16 April 2012. We present regular observations from the Centro Nacional de Prevención de Desastres (CENAPRED, based in Mexico City) as well as results from remote sensing efforts (thermal and ash cloud detection methods). Few ash plumes rose over 2 km above the ~5.4 km summit but ash fell as far as 70 km from the summit.

Ash detection. The Washington Volcanic Ash Advisory Center (VAAC) reported ash plumes every month from December 2011 through June 2012 except for February (figure 61). Specifically, although seven notices were released in January 2012, no alerts regarding potential or observed ash were issued again until 30 March 2012, when ash was reported drifting NE. The alerts, Volcanic Ash Advisories (VAAs), often described "exhalations" and "discrete" plumes based on reports from CENAPRED and detection by the GOES-13 weather satellite. Ash was detected at altitudes ranging from 4.5 to 11.5 km with various drift directions.

Figure 61. On 25 January 2012, the Washington VAAC released the mapped area and text details of observed airborne ash from Popocatépetl. A "short exhalation of ash" had reached an altitude of ~11 km. VAAC reports with graphics showing polygons that enclose inferred ash drift have been available for years, but those on Google Earth backgrounds are relatively new. Courtesy of the Washington VAAC.

Activity escalated and peaked in April 2012. This report continues the table in the previous report (BGVN 36:11) that tallies ash emissions, and includes events such as incandescence and elevated seismic activity (table 22). Incandescence and numerous plumes of gas and ash were reported from December 2011 to April 2012. Events escalated later, on 13 April, when seismicity increased and explosions occurred at the summit. Blocks and incandescent ejecta exploded from the crater and fine ash from these events was reported in nearby towns. On 16 April CENAPRED announced that hazardous conditions had escalated and increased the Alert Level at the volcano from Yellow Phase 2 to Yellow Phase 3. Local news reported that the governor of Puebla, Rafael Moreno Valle, had announced that schools located within 12 km of Popocatépetl were closed. Significant ash emissions continued from April through June. On 18 April an explosion sent incandescent fragments over 800 m from the crater; that material covered snowy areas near the summit and formed small lahars (table 22). On 8, 10, and 12 May local news reported that the Puebla airport was closed due to volcanic ashfall.

Table 22. Emissions and activity from Popocatépetl's summit crater between 14 December 2011 and 26 June 2012. Data provided by the Centro Nacional de Prevención de Desastres (CENAPRED).

Figure 62. (top) Night glow and ejecta on the N flank of Popocatépetl on 14 May 2012; image taken at 0508 by the Tlamacas camera, which is located ~5 km N of the edifice. (bottom) A persistent gas-and-steam plume taken by the Altzomoni camera, located ~10 km to the NNW of Popocatépetl, at 1053. Courtesy of CENAPRED.

MODVOLC Thermal Alerts. Elevated temperatures from the summit area of Popocatépetl have been detected regularly by satellite remote sensing, in particular, using the Geostationary Operational Environmental Satellite (GOES) and Earth Observing System (EOS) satellites (Wright and others, 2002; Wright and others, 2004). The ability to detect elevated temperatures varies significantly based on meteorological changes, cloud cover, and solar irradiance; volcanic explosions, however, have produced distinctive radiance signatures and have been reliably documented (Wright and others, 2002).

A study focused on 1998 thermal data from Popocatépetl highlighted the challenges unique to the remote sensing of lava domes (Wright and others, 2002). Investigators correlated the irregular appearance of thermal anomalies with physical changes at the dome. Seismic and SO2 gas flux data supported the conclusion that high thermal emissions followed explosions, "which served to disrupt the cool dome carapace and expose the much hotter interior to the orbiting sensor. Thus, it was not the presence of the dome itself that produced elevated levels of radiance but rather transient processes affecting its surface temperature."

A primary tool for monitoring volcano thermal emissions is the MODVOLC algorithm which was developed by the Hawaii Institute of Geophysics and Planetology (HIGP), in operation since 2002. The algorithm flags thermal anomalies that appear in data captured by the MODIS sensor (on EOS satellites Terra and Aqua) (Wright and others, 2004). The online archive of graphic and text results was designed to be updated, typically within 12 hours of each MODIS overpass. Since local fires, meteorological conditions, and eruption clouds influence the detection of thermal anomalies and cannot be quantified, investigators emphasize caution and awareness of local conditions when interpreting short-term (for example, day-to-day) thermal anomaly data at individual volcanoes.

From June 2011 through June 2012, ~230 thermal alert pixels were detected near the summit area of Popocatépetl with the MODVOLC system (figure 63). From October 2010 to June 2012, thermal anomalies were reported every month except for August 2011 (table 23). Generally 1-2 anomalies were detected in each report; on rare occasions 3-4 anomalies were detected. While CENAPRED has reported numerous ash and explosive events at the summit during this time period, meteorological conditions may have reduced thermal data acquisition during the rainy seasons (May through September).

Figure 63. A) Locations of the Trans-Mexican Volcanic Belt (TMVB) including the location of Popocatépetl with relation to surrounding populated areas (Cross and others, 2012). B) Hotspots detected at Popocatépetl with the MODVOLC method from June 2011 through June 2012. Grid lines are 10 km between intersections. Each thermal pixel is one square kilometer. Pixels were concentrated around the summit region with one anomalous hotspot of clear non-volcanic origin located ~20 km SW. Courtesy of HIGP.

Table 23. Monthly summary of thermal anomalies detected at Popocatépetl from October 2010 through June 2012. The number of reports for each month is based on the daily summaries available on the MODVOLC website and the number of pixels was counted from each report. Only pixels within a 10 km radius of the summit were included in this table. Courtesy of HIGP.

Since our last report discussing ash plumes and increased seismicity noted in April 2012 (BGVN 37:05), ash plumes from Popocatépetl's summit continued to be emitted at a reduced rate during July-October 2012. During this reporting period, the Centro Nacional de Prevención de Desastres (CENAPRED, based in Mexico City) noted persistent incandescence and gas emissions (sometimes containing ash). Seismicity decreased significantly in August and, on 1 September, CENAPRED reduced the Alert Level from Yellow, Phase Three to Yellow, Phase Two. New volcanic hazard maps were available from investigators focused on ejecta from the summit (Alatorre-Ibargüengoitia and others, 2012) and CENAPRED developed an interactive web interface for compiling layers of hazard zones based on volcanic phenomena expected from Popocatépetl.

Visual observations July-October 2012. From July through October 2012, CENAPRED reported that cloudcover frequently obscured the view of Popocatépetl's summit. During cloud-free conditions, incandescence from the crater could be observed at night and early in the morning. Moderate explosions (many containing incandescent tephra) occurred almost daily.

Several larger explosions were observed by CENAPRED on 21 July, in August (7, 17, 18, 20, 26 and 27), September (10, 14, and 15), and October (17, 18, 20, and 26). These events were captured by web cameras when incandescent tephra was ejected and traveled up to 1.5 km from the summit (figure 64).

Figure 64. On 20 October 2012, the webcamera Altzomoni (located ~10 km to the NNW) captured moderate-sized explosions from Popocatépetl that ejected incandescent tephra across high elevation areas (within 1 km of the summit). From left to right, photos were captured at 05:34:25, 05:34:41, 07:24:23, and 09:36:44. Courtesy of CENAPRED.

When conditions permitted during July-October, gas-and-steam plumes (frequently containing ash), were observed reaching 0.5 to 2.5 km above the crater. Asfall was reported in the community of Ozumba (18 km W) on 21 July 2012. CENAPRED reported that an ash plume rose 4 km above the crater at 1758 on 6 August; incandescence from the crater was also observed that day.

Two lahars were detected in July 2012. The event on 3 July occurred at 1530 and was documented by the Tlamacas web camera (located ~5 km N of the crater). On 12 July, between 1938 and 2135, a lahar occurred on the N flank. Both of these events were attributed to glacial melt high on the flanks of Popocatépetl; no flooding was reported at low elevations.

VAAC ash detection during July-October 2012. The Washington Volcanic Ash Advisory Center (VAAC) announced observations of intermittent ash plumes from Popocatépetl during July-August 2012. At least five announcements were based on CENAPRED reports and imagery from local web cameras; cloud cover frequently obscured remote sensing images. During July-August, plume altitudes were in the range of 6.4-9.1 km (0.97-3.67 km above the crater); plumes tended to drift up to 130 km to the W, S, and SE.

No VAAC reports were released in September 2012 and five reports of observed ash were made in October 2012. During 11-26 October, maximum altitudes of ash plumes reached 7.6 km (~2.2 km above the crater) and tended to drift W, NW, and S.

Seismicity during July-October 2012. Decreasing seismicity was detected at Popocatépetl between July and October 2012. Approximately 30 hours of tremor were recorded by CENAPRED in July and ~18 hours in August, while approximately 9 and 10 hours were recorded in September and October, respectively. Volcano-tectonic (VT) earthquakes also occurred less frequently between July (~53 detected) and September (four detected). In October, ~15 VT events were reported in CENAPRED's online reports.

Hazard map for volcanic ejecta. A recent investigation by Alatorre-Ibargüengoitia and others (2012) highlighted the frequently-occurring Vulcanian eruptions of Popocatépetl and developed a volcanic ejecta risk assessment. Volcanic bombs have impacted the immediate summit area of the volcano as well as locations as distant as 3.7 km (the maximum distance considered in the simulations). The investigators combined video observations of past eruptions, field studies (between 1999 and 2010), and a ballistics model designed for simulating optimal launching conditions. One of the results from this investigation was a map defining three risk zones (figure 65).

Figure 65. This map of Popocatépetl's summit, flanks, and local infrastructure includes hazard zonation for volcanic bombs. Locations with 5-point stars represent observed impact sites from 1998-2006; the 4-point star represents an impact site dating to 14,000 years before present. From Alatorre-Ibargüengoitia and others (2012).

An interactive hazard map for Popocatépetl was available online through the CENAPRED website (figure 66). Basemap options included terrain, streets with major towns, and satellite imagery. Users were able to choose from various volcanic phenomena (lava flows, ash fall, lahars, and pyroclastic flows) to view the predicted aerial extent of the relevant hazard. The flow paths and inundation areas for lava flows, lahars, and pyroclastic flows were determined with TITAN2D software based on data collected by CENAPRED. The zones representing high (red), medium (orange), and low (yellow) risk of ashfall (note the concentric zones in figure 66) were developed based on a 1995 study by CENAPRED (Macias and others, 1995).

Figure 66. This map is a screenshot of CENAPRED's interactive hazard map, showing results from four scenarios; three types of lahar inundation parameters (highlighted drainages within ~40 km of the summit) and zones defining potential ashfall. Note that lahar hazards are within the orange zone of moderate ashfall hazards; the yellow zone indicating minor ashfall reaches Mexico City. The radius of the yellow zone (minor ashfall) is ~70 km. Courtesy of CENAPRED.

During November 2012-December 2014, ongoing exhalations and explosions

Introduction. This report summarizes events at Popocatépetl during November 2012-December 2014. Almost all of the data discussed came from (~800) online daily reports by the Centro Nacional de Prevención de Desastres (CENAPRED). Many of those reports are issued covering a 24 hour interval (from 1000 on the stated day back to 1000 on the previous day), with occasional cases of later supplemental reports the same day. A link to those reports is provided in the "Information Contacts" section. Our previous report on Popocatépetl discussed the ongoing eruption during July-October 2012 (BGVN 37:09).

Behavior during the reporting interval included persistent emissions (often containing ash). When visibility permitted, web cameras documented nighttime emissions containing incandescent fragments, in many cases, rising hundreds of meters above the crater rim and spreading across the upper flanks. These eruptions typically deposited tephra up to ~1.5 km from the crater where it was conspicuous on the snow and ice that crowns the summit. Occasional air photos also depicted ballistics or their impacts and tracks in the summit area. Ashfall was not uncommon in villages on the volcano and it occasionally fell in parts of Mexico City and the city of Puebla. Many plumes rose on the order of 1 km, reported by CENAPRED in many cases several times a week if not more frequently. Periods of tremor occurred, some of which lasted for more than one hour. At least one volcanic-tectonic earthquake occurred on many days (maximum coda magnitudes, Mc, generally 2.0 to 2.5). Earthquakes are in general thus dismissed from detailed discussion below; however, for one sample month, November 2013, we include a larger emphasis on the record of larger earthquakes reported daily by CENAPRED. Many of the commonplace processes such as those in the above list were sufficiently common that, in order to save space, they are often omitted from this narrative.

One way CENAPRED quantifies Popocatépetl's behavior is to use daily 'exhalations' (substantive plumes inferred to contain ash) which have long been a means of monitoring and characterizing this large and tall andesitic stratovolcano. The term 'exhalation' was used extensively in Bulletin reports starting with BGVN 22:03 in 1997. Exhalations are still currently tabulated by CENAPRED. Those appear in histograms in each daily report (assessing a 24 hour interval ending at 1000 on the stated reporting date).

Wright and others (2002) explain 'exhalations' further and clarify the distinction to the larger events that they classify as 'explosions.' The authors included photos and infrared imagery to illustrate the term (omitted here).

"Exhalations are short duration (3–90 min) ash-rich gas plumes . . .. CENAPRED provide daily Web-based activity updates in which exhalations are classified as small, moderate, or large on the basis of their duration and resultant plume height. Plumes can rise as much as 5000 m above the crater rim but are generally smaller. Exhalations are common and as many as several tens can occur each day. The ash they transport may be non-juvenile in nature (possibly with a juvenile component since March 1996 when lava extrusion began), and exhalations are thought to be the result of intermittent high-pressure gas streams that scour rock fragments from the conduit walls. Thermal video images, which measure the amount of radiation emitted in the 8–14 μm region of the electromagnetic spectrum . . ., indicate that by the time the plumes have reached the altitude of the crater rim, the ash-gas mixture is generally of a very low temperature (9–12°C at the plume exterior) due to the rapid entrainment of air at ambient temperatures.

"Explosions are less frequent than exhalations. They result in larger, darker ash plumes, with bombs often thrown clear of the crater to form a high-temperature ejecta blanket on the upper slopes of the volcano . . .. The plumes most commonly reach heights of between 3000 and 5000 m above the crater rim, although several larger explosions have occurred during the recent activity. The explosion of 30 June 1997, for example, was the largest recorded since 1922 and generated a plume 13,000 m high. Although explosions during the recent activity have been most common during periods of dome growth, they have also been observed during periods when no magmatic activity has been observed on the crater floor."

Wright and others (2002) also make this comment: "Clearly, periods of prolonged and total cloud cover will prevent any useable data being acquired." They were addressing satellite observations but this also applies to visual- and webcamera-based observations. This means that during some intervals adverse meteorological factors (clouds, rain, snow, etc.) could reduce the number of reported exhalations.

From this it is reasonably clear that the vast majority (if not all) of the eruptions during the reporting interval (November 2012-December 2014) were in the category of exhalations. During this reporting interval, several plumes did reach 3-4 km above the crater rim, as is noted below (e.g., during May-July 2013) and but we know of none reported that rose to over 5 km over the crater rim (~10.4 km altitude).

The maximum number of daily exhalations in the recent past stood, since July 2012, at 211. On 23 May 2013 that record was broken when 314 daily exhalations occurred. A second increase in that maximum value occurred twice more when the daily values reached 480 exhalations on both 4 and 6 June 2014.

As discussed in other Bulletin reporting since the onset of the eruption in March 1996 (BGVN 21:01), dramatic events involved dome dynamics in the steep-walled, cylindrical, ~0.5-km-diameter summit crater. There, emissions of lava and tephra constructed the dome. Occasional energetic discharges from the vent beneath this growing dome blew out the dome's central area, leaving the dome with a ring-shaped morphology. This process has taken place many times in the intervening years since 1996 and continued in this reporting interval too.

Further discussion and references on the topic of exhalations and explosions with particular reference to Popocatépetl also appear in other studies (e.g., de la Cruz-Reyna and others, 2008; González-Mellado and de la Cruz-Reyna, 2008; and Tárraga and others, 2012).

November 2012-December 2013 activity. During the remainder of 2012, the Alert Level remained at Yellow, Phase Two (where it had been since lowered on 1 September 2012).

The usual plumes, occasionally bearing ash, rose up to ~1 km above the crater on many days during November-December 2012. For example, during 3-4 November 2012, CENAPRED daily reports noted 9 more significant eruptions and associated plumes registered at these respective times: 1100, 1450, 1548, 2346, 0157, 0240, 0532, 0835, and 0931.

On the basis of 15-day averages shown on histograms in CENAPRED daily reports, the overall November monthly average was 43. During December 2012 the overall average was 31. Lower monthly averages than December's 31 appeared during January 2013 through the first half of March 2013. During the second half of March 2013 the average daily exhalations again rose to similar levels (31). The averages dropped again after that the averages remained low well into May and early June 2013 although during these later months some daily values increased significantly. The average value for the second half of June 2013 was 33.

During the first two weeks of May 2013 there were increases in earthquakes, tremor, and emissions. During 7-8 May, CENAPRED called attention to an episode of high amplitude spasmodic tremor. It was accompanied by an explosion on 8 May that ejected an ash plume that rose 3 km above the crater and drifted SE. Ashfall was reported from the villages of San Pedro Benito Juarez (10-12 km SE), San Juan Tianguismanalco (22 km SE), and Atlixco (23 km SE), and in some areas of the City of Puebla (~50 km to the E). The main tremor episode was accompanied by incandescent fragments that reached up to 500 m distance from Popocatépetl (chiefly NE). As reported on the 8th, during the last 24 hours CENAPRED detected 40 low intensity exhalations; 2 additional stronger ones sent a small amounts of ash towards the SE. Tremor during early May generally remained below daily intervals of up to a few hours.

On 10 May 2013 CENAPRED noted that during the last 24 hours there occurred 46 generally small exhalations. In addition, two explosions occurred, of moderate magnitude, sending ash ~1 km above the crater. Tremor duration for that interval lasted ~3 hours, including some time periods with high-amplitude signals. Three small volcano-tectonic earthquakes also occurred. A second report later on 10 May indicated that during 1142-1443 a series of ash emissions and periods of spasmodic and harmonic tremor occurred with ash plumes rising as much as 1 km above the crater, again producing ashfall. Similar plume heights were seen on 11 May, and the daily report noted there were in the last 24 hours a total of 53 (chiefly small-to-moderate) exhalations.

According to CENAPRED, seismicity had intensified on the afternoon and night prior to 12 May (when the Alert Level rose to Yellow, Phase Three, stipulating a 12 km radius exclusionary zone). Additionally, the report for 12 May 2013 said that in the last 24 h, 43 exhalations of low and moderate intensity were recorded. In general, steam-and-gas plumes with small amounts of ash rose from the crater. Although foggy conditions sometimes limited visibility, sporadic ejections of incandescent tephra fell back into the crater and onto the NNE flank, 300 m from the crater rim. Tremor registered in 1-2 hour intervals, continuously or in segments. Each such interval began with an eruptive burst of moderate intensity. The most important burst took place at 1700 on the 12th and was perceived by many residents in the E and SE sectors.

On 13 May 2013 steam-and-gas plumes were observed rising from the crater during periods of good visibility. On 14 May an explosive event generated an ash plume that rose to 3 km altitude. Incandescent tephra landed up to 600 m away on the NE flank. Cloud cover again obscured summit views. Seismicity, including tremor, remained elevated. The histogram in the daily report listed 25 exhalations during the past 24 hours.

On 14 May 2013, volcanologists aboard an overflight observed a lava dome 350 m in diameter and 50 m thick, found the dome slightly deflated after an explosion. Similar dome- related events seemingly took place again during the next few days. The histogram in the daily report listed 41 exhalations during the past 24 hours.

CENAPRED noted a vigorous eruption at 0146 on 15 May that discharged an ash plume to over 3 km above the crater rim, blown NE sending tephra up to 1.5 km downslope. At 1804 that day a second blast sent a column to somewhat below 3.5 km above the crater, blown N. Both these events correlated with spasmodic tremor. The histogram in the daily report listed 56 exhalations during the past 24 hours.

On 16 May 2013, some intervals of tremor again corresponded with discharge of glowing fragments, the majority of which fell back into the crater (a process frequently mentioned throughout the reporting interval). Ash plumes rose 2 km and drifted NE. Minor ashfall was reported in Paso de Cortés, 7 km N. Incandescent tephra reached 400 m from the crater rim to the N and NE. Seismometers registered an Mc 2.2 earthquake. The histogram in the daily report listed 55 exhalations during the past 24 hours.

Two punctuated eruptions were described for the 24-hour interval ending at 1000 on the 17th (one reaching 4 km above the rim) The first took place at 2214, when the crater issued a strong explosion; the resulting incandescent fragmental material covered the flanks to 1.5 km distance and the associated gas-and-ash column rose to under 3 km above the crater, drifting NE. The second took place at 0028, generating an eruption column to 4 km above the crater and casting glowing fragments up to 1.5 km from the crater. The report for the 17th said that moderate-to-small exhalations during the past 24 hours totaled 31. On 17, 18, 19, and 20 May 2013 histograms in the respective CENAPRED reports noted that in the past 24 hours they each registered 31, 18, 24, and 54 exhalations.

During an overflight on 18 May, volcanologists observed the active crater, 200 m wide and 40 m deep, located in the dome's surface. The rest of the dome was covered with rock fragments. Tephra had landed as far as 0.5 km down the NE flank. CENAPRED inferred that the missing material forming this crater was likely excavated by explosions associated with hours of tremor that took place during 14-17 May.

On 23-27 May 2013, tremor decreased. A flight on 28 May captured several photos, one of which appears in figure 67. Note the steep crater within the ring-shaped dome and the abundance of fragmental character of some material on the dome's surface. The CENAPRED caption also drew attention to marks made by ballistic material that burrowed into the snow and ice in the summit area.

Figure 67. Aerial photo taken looking downward at the summit area of Popocatépetl on 28 May 2013. The summit hosts a deep, steep sided circular crater, within which grows a dome. The dome is frequently reamed out by powerful explosive bursts leaving the dome with a crater as seen here. Courtesy of CENAPRED (from their 1 June 2013 daily report).

During 1-7 June exhalations on the daily histograms ranged between 32 and 93. They were often described as of low intensity (steam rich and ash poor), but in some cases they were described as reaching moderate intensity. Cloud cover often prevented visual observations. Volcano-tectonic earthquakes up to Mc 2.7 took place. On 7 June 2013 the Alert Level was lowered to Yellow, Phase Two.

During the rest of June 2013, significant emissions continued. For example, during 12-17 June 2013, plumes containing ash rose as high as 4 km above the crater, and ashfall was reported in many nearby villages (figure 68). For the eruption on the 17th, perceptible ash fell as far as the SE portion of Mexico City. The eruption on the 17th was accompanied by tremor with a duration of over 2 hours and other seismicity also remained at times high.

Figure 68. Webcam image of an explosion at Popocatépetl on 17 June 2013 (at 13:26:55 local time, which corresponds to 18:26:55 UTC). The explosion generated an ash plume that reached greater than 4 km above the crater and threw incandescent fragments up to 2 km out of the crater, causing small grassland fires. Courtesy of CENAPRED.

An overflight on 25 June led to the insight that eruptions in the past few days had further altered the dome. It then had the dimensions of 250 m in diameter and 60 m deep.

According to CENAPRED's daily report on 3 July 2013, seismic activity increased again during the past 24 hours when the seismic network detected tremor for 36 minutes and two larger earthquakes (at 0407 and 0918 on the 3rd) with respective coda magnitudes, Mc 2.9 and 2.6. The daily report noted 84 exhalations on the part of the histogram for the last 24 hour interval ending at 1000 on 3 July. This was accompanied by persistent gas and ash emissions and diffuse ash plumes that rose 2-3.5 km above the crater and produced ashfall in areas as far as México City. Incandescent tephra was ejected short distances onto the N and E flanks.

This increased activity continued on 4 July 2013. According to news articles, multiple airlines canceled flights to and from the México City and Toluca (105 km WNW) airports on 4 July. The number of cancelled flights, according to the news, was 47. Flights resumed later that day.

On 5 July 2013, almost continuous tremor was recorded. Ash plumes drifted NW. Scientists employed both infrared webcamera imaging and an overflight to observe continuously ejected incandescent tephra that landed as far away as 1.5 km from the crater on almost all flanks, and an ash plume that rose 2 km. Cloud cover often obscured visual observations. A news article stated that four airlines canceled a total of 17 flights.

On 6 July 2013, low frequency, high amplitude tremor was accompanied by gas, steam, and ash emissions that rose 3 km. Three explosions were detected, but cloud cover prevented visual confirmation. News articles noted ashfall again in parts of México City. Government officials raised the Alert Level to Yellow, Phase Three, excluding the public within a 12 km radius of the crater. Later that day, the low frequency tremor amplitude decreased, followed by diminishing emissions of gas and ash.

During 7-9 July 2013, tremor was accompanied by persistent emissions of steam, gas, and small amounts of ash that drifted WSW and NW; cloud cover continued to hinder visual observations. Three explosions produced gas containing ash. Incandescence and ejected incandescent tephra were sometimes observed. During overflights on 7 and 10 July, scientists observed that a new lava dome, 250 m in diameter and 20 m thick, had recently formed in the crater.

During an overflight on 15 July 2013, scientists observed a 200-m wide and 20 to 30 m deep crater in the lava dome. The attributed the new morphology last seen on 10 July to dome destruction owing to explosions in the past few days. They also reported on M 2.3, 1.8 and 1.7 earthquakes, as well as 82 minutes of high-frequency tremor on 15 July 2013.

Emissions and occasional explosions that generated plumes with some ash continued during 10-16 July 2013 (figure 68). According to a news article, on 12 July 2013 an Alaska Airlines flight to México City's international airport was canceled and operations at a small airport in Puebla were suspended.

On 23 July 2013, the Alert Level was lowered to Yellow, Phase Two, a status that prevailed through December 2014 (the end of this reporting interval).

On 31 July 2013 a clear decrease in the size of the water vapor and gas plumes was observed; plumes blew down the NW flank and rose only 100 m above the crater rim. An explosion was detected at 2312 on 1 August, but cloud cover prevented confirmation of any ejecta. On 2 August minor amounts of ash fell in the Tepetlixpa, Atlautla, Ecatzingo, and Ozumba municipalities of Mexico State. On 4 August emissions of gas, steam, and ash drifted NW. During 5-6 August a few observed plumes rose 1-2 km and drifted WNW, W, and WSW.

On 14 August 2013 a period of tremor was accompanied by an ash emission that drifted W and fell on towns as far as ~20 km away. Gas-and-steam plumes were observed during 15-16 August. A period of tremor on 17 August was accompanied by an ash plume that rose 1.5 km and drifted WSW. Ash fell in in towns as far as 65 km SW (Cuernavaca). On 18 August tremor was accompanied by an ash emission that rose 1.2 km and drifted SW. On 19 August minor steam-and-gas emissions drifted W. During 19-20 August emissions likely contained small amounts of ash but cloud cover prevented confirmation. On 28 August ash plumes rose 200-800 m and drifted SW. Gas-and-steam plumes were observed the next day and on the 30th an ash plume rose 1 km above the crater and drifted W.

During much of September and October 2013 clouds sometimes blocked clear views of the volcano. The volcano continued to undergo seismic unrest and to emit steam and gas plumes often containing minor amounts of ash. Early September ash blew WSW to fall on settlements as far as 24 km away (including, on the 1st, at Tetela del Volcán, 20 km SW, and Ocuituco, 24 km SW, and on the 2nd, at Ecatzingo, 15 km SW. On 4 September the number of daily exhalation during the previous 15 days averaged at 5, but on that day it stood at 44 exhalations. Average values for 15 day intervals remained under ~25 during September through December 2013.

Other observational details from this interval are similar to those noted above. For example, on 24 October an explosion at 2111 produced an ash plume that rose 1 km and drifted SW. Eight low-intensity explosions on 26 October increased gas and steam emissions and produced slight amounts of ash.

Despite the low number of exhalations near year's end, during 30 October 5 November 2013, exhalations were frequently detected, varying from 30 to 97 times per day. Between 31 October and 5 November, four volcano tectonic earthquakes were recorded (Mc 2.1-2.5). Tremor was frequently detected; on 1 November, 3 hours and 21 minutes of high-frequency tremor was recorded.

The Washington Volcanic Ash Advisory Center (VAAC) issued advisories for Popocatépetl every month during 2013, except for November and December 2013. The advisories were most numerous during April through July 2013. According to CENAPRED, a daily average of about 6,000 metric tons of sulfur dioxide was emitted during both 2013 and 2014.

2014 activity. During 2014, the Washington VAAC issued advisories for Popocatépetl every month, except for March. The number of advisories issued was considerably lower than that for 2013. At year's end, the Alert Level remained at Yellow, Phase Two.

Activity in 2014 was broadly similar to that in 2013, with above-mentioned frequent gas-and-steam emissions, often with minor ash content. Issues with limited visibility at times due to cloud cover also remained.

The 15-day average of the daily exhalations often stood at less than 4 during January and through 19 February 2014. Activity increased during 19-25 February 2014. At least eight explosions generated plumes (mostly ash) that rose 1-2 km above the crater. An explosion at 1233 on 21 February sent an ash plume to 4 km above the crater rim. On 26 February, scientists aboard an overflight observed that another lava dome (dome number 48) had been destroyed, leaving a funnel shaped cavity about 80 m deep. A new dome 20 30 m wide was at the bottom of the cavity. On 27 February, activity decreased considerably.

CENAPRED's 15-day average of the daily exhalations stood at 7 or below during March 2014 but it rose to 34 by 18 April 2014 and dropping to 22 by the end of that month. It rose again in late May 2014 (to 45 on 31 May). On 16 June it stood at 57; and for the last half of June it declined to below 10.

The daily value reached 480 exhalations on 4 and 6 June 2014, a new record.

Monitored and eruptive activity briefly increased in early July 2014. For example, CENAPRED reported tremor on 2 July (maximum of 80 minutes in 24 hours) and 12 July (minimum of 8 minutes). Up to 216 exhalations of low and moderate intensity were detected on 9 July. The 15-day average of the daily exhalations also rose during early July 2014, reaching over 50 during the first half of the month but dropping towards the end to 15 (on the 31st).

The first half of August had a 15-day average of 33 daily exhalations and the second half, 46 daily exhalations. Those averages (first half of the month and second half of the month) were as follows for the rest of the year: September (14 and 31); October (40 and 39); November (55 and 13); and December (41 and 72).

During 27 August and 2 September 2014, plumes reached as high as 3 km above the crater. Tremor and volcanic-tectonic earthquakes were recorded in early September.

On 17 September 2014, a day with 126 exhalations recorded by CENAPRED's monitoring system, an ash emission at 1813 resulted from a moderate explosion. The emission reached 3 km above the crater rim. It blew SE and light ash fell at villages in that direction. During the same day five other exhalations reached ~1.5 km above the crater rim. During 7-8 October 2014 ashfall was reported in Cuautla (43 km SW), Tetela del Volcán (20 km SW), Huaquechula (30 km SSW), and Morelos (60 km SW). On 12 October ash plumes rose 2 km and drifted NE. Ashfall was reported in Paso de Cortés (8 km NNW) and Tlalmanalco (30 km NW).

CENAPRED reported that during a 14 October 2014 overflight, volcanologists observed that the diameter of the inner crater (formed in July 2013) had increased to 350 m. The bottom of the inner crater floor was 100 m below the floor of the main crater, cup shaped, and covered with tephra. No sign of the lava dome (number 52) emplaced in early August 2014 was visible. Steam emissions originated from a crack in the N wall of the inner crater and ash emission came from the bottom of the crater.

Although for brevity we have generally excluded examples of explosions and ashfall for September and October 2014, which were broadly similar to previous months, a small explosion at 0317 on 25 October ejected tephra 100 m outboard onto the S flank. A steam-and-gas plume containing a small amount of ash rose 1.5 km above the crater and drifted SW. Ashfall was reported in Tetela del Volcán (20 km SW). A small explosion at 0111 on 26 October ejected a plume that rose 1.1 km above the crater rim and sent tephra 200 m onto the N flank.

Histograms in daily reports issued during 3-5 November 2014 described exhalations totaling 267, 190, and 147, respectively. These were broadly described as a continuous gas-and-steam plume, mainly with minor amounts of ash. Some more vigorous and ash rich emissions occurred and, for example, on 5 November the plume rose as high as 1 km and caused light ashfall in Paso de Cortés. That daily report also showed videos that showed incandescent fragments spreading ~800 m over the upper flanks. The 5 November report also showed a seismic record captured in the interval 2000 on the 4th to 0130 on the 5th illustrating ~190 seismic events. About an hour after those events, the same record indicated an Mc 2.1 earthquake. On 6 November, a small rockslide on the SW flank was recorded by a webcam and the seismic network. Scientists aboard an overflight observed a new dome (number 53), emplaced during 4 5 November; it was an estimated 250 m in diameter and 30 m thick.

During 7 11 November 2014, seismicity indicated continuing gas-and-steam emissions, with small amounts of ash. Incandescence from the crater was observed most nights. Explosions during 10-11 November ejected incandescent tephra and generated ash plumes that rose 1.2 km above the crater. Gas-and-steam emissions continued through the rest of November.

During December 2014, occasional explosions continued, generating ash plumes that rose as high as 3-3.5 km, resulting in minor ashfall on nearby villages. One or more rockslides were noted in addition to the usual small ash plumes, the occasional incandescence at the crater and associated with tephra. One plume on 8 December rose to 3 km above the crater. CENAPRED reported that the international airport in Puebla was temporarily closed on 17 December 2014 due to ashfall from an explosion that generated a 2 km high ash plume. The explosion also ejected incandescent tephra that landed 700 m down the N flank. During an overflight during the last half of December, volcanologists observed a lava dome at the bottom of the crater. The Alert Level remained at Yellow, Phase Two.

González-Mellado, AO, and de la Cruz-Reyna, S, 2008, A simplified equation of state for the density of silicate hydrous magmas: an application to the Popocatépetl buoyancy-driven dome growth process. Journal of Volcanology and Geothermal Research, 171(3), 287-300.

Ash plumes several times weekly, multiple episodes of dome growth and destruction, and high SO2 flux during January 2015-June 2016.

Frequent historical eruptions, first recorded in Aztec codices, have occurred since pre-Columbian time at México's Popocatépetl, the second highest volcano in North America. More recently, activity picked up in the mid-1990s after about 50 years of quiescence. The current eruption, which has been ongoing since January 2005, has included frequent ash plumes rising generally 1-4 km above the 5.4-km-elevation summit, and numerous episodes of lava-dome growth and destruction within the 500-m-wide summit caldera. Multiple ash emissions generally occur daily, with larger, more explosive events that generate ashfall in neighboring communities occurring several times each month. Information about Popocatépetl comes primarily from daily reports provided by México's Centro Nacional de Prevención de Desastres (CENAPRED). Many ash emissions are also reported by the Washington Volcanic Ash Advisory Center (VAAC). Satellite visible and thermal imagery and SO2 data also provide important information about the character of the eruptive activity. Our last report covered activity through December 2014 (BGVN 40:02); this report covers 2015 and the first six months of 2016.

CENAPRED reported near-constant emissions of water vapor, gas, and minor ash during 2015 and January-June 2016. Ash plumes from larger explosions regularly occurred several times per day during the more active months, and a few times a week during the quieter months. Ashfall is sometimes reported within 40 km of the summit. The plumes generally rose to altitudes of 6.1-7.9 km, and occasionally higher. The prevailing winds most often sent the ash NE or E, but multi-direction plumes at different altitudes were also common. Incandescent tephra was ejected onto the flank within 1 km of the summit every month, and was reported 3.5 km from the summit after stronger activity on 3 April 2016. Sulfur dioxide emissions are persistent, with plumes drifting a hundred or more kilometers from the volcano observed regularly in satellite data. Two episodes of dome growth were reported in February and April 2015, and dome destruction was inferred during January 2016.

Activity during January-June 2015. During January 2015 CENAPRED reported at least 13 explosions with ash-bearing plumes, as well as near-constant emissions of water vapor and gas that sometimes contained ash. The ash plumes generally rose to 600-1,500 m above the summit crater (up to 6.9 km altitude) and drifted either E or NE. Incandescence from the crater was visible on most clear nights. The Washington VAAC issued two series of reports; ash emissions on 4 January were not observed in satellite imagery due to weather clouds, but the 17 January emission was observed via webcam and satellite images at 5.8 km altitude drifting E. There were 58 MODVOLC thermal alerts issued in January, all from the immediate vicinity of the summit crater; most days had multiple-pixel alerts. NASA's Global Sulfur Dioxide monitoring system captured nine days of SO2 emissions with values greater than two Dobson Units (DU), a measure of the molecular density of SO2 in the atmosphere. Values greater than 2 show as red pixels on the imagery created from the OMI on the Aura satellite (figure 69).

Figure 69. Sulfur dioxide plume from Popocatépetl on 15 January 2015 extending ENE from the summit over the Gulf of México. The gas is measured in Dobson Units (DU), the number of molecules in a square centimeter of the atmosphere. If you were to compress all of the sulfur dioxide in a column of the atmosphere into a flat layer at standard temperature and pressure (0° C and 1013.25 hPa), one Dobson Unit would be 0.01 millimeters thick and would contain 0.0285 grams of SO2 per square meter. The red pixels represent values >2 DU. Courtesy of NASA Goddard Space Flight Center (GSFC).

The volcano was very active during February 2015. CENAPRED reported that their seismic network recorded several hundred low-intensity events that were accompanied by steam-and-gas-emissions and usually contained ash. Numerous explosions were attributed to lava-dome growth. Ash plumes rose 1-2 km above the crater, generally drifting NE. Ashfall was reported a number of times in communities up to 50 km away, and incandescence at the summit was observed on many nights.

On 11 February, ashfall was reported in the city of Puebla (~50 km to the E) and in the municipalities of Juan C. Bonilla (30 km ENE), Domingo Arenas (22 km NE), Huejotzingo (27 km NE), and at the airport to the E. On 15 February, explosions generated ashfall in Huejotzingo, Domingo Arenas, Salvador el Verde (30 km NNE), San Felipe Teotlalcingo (26 km NNE), and Puebla. Five explosions generated ash plumes on 18 February (figure 70). On 21 February, there were 22 small explosions, some of which ejected tephra 200 m onto the NE flank. A series of explosions on 24 February ejected incandescent material as far as 700 m onto the NE and SE flanks.

Additional explosions (19) detected on 25 February resulted in ashfall 20-37 km to the NE in San Martín Texmelucan (35 km NE), San Matías Tlalancaleca (35 km NE), San Salvador el Verde (29 km NE), Santa Rita Tlahuapan (34 km NNE), Tlaltenango, Huejotzingo, San Miguel Xoxtla (37 km NE), Domingo Arenas, Santa María Atexcac (20 km NE), and the Puebla airport (30 km NE). Explosions on 26 February ejected incandescent tephra 700 m onto the N and NE flanks; ashfall was again noted in Domingo Arenas, San Martín Texmelucan, and Huejotzingo in the state of Puebla. The international airport in Huejotzingo suspended operations to clean up the ash. On 27 February explosions generated ash emissions and again ejected incandescent tephra 300 m onto the flanks. Ashfall was reported in Huejotzingo, Domingo Arenas, Tlaltenango, San Andrés Cholula (33 km E), and Puebla. Two separate series of explosions were detected on 28 February, and more incandescent tephra was ejected 300 m onto the flanks.

During an overflight on 17 February, volcanologists observed a dome at the bottom of the inner crater, which formed in July 2013 and extends 100 m below the floor of the main crater. They identified this as dome number 55; it was 150 in diameter. On a second overflight on 27 February, volcanologists observed that the dome had grown and was filling the bottom of the inner crater (figure 71). The dome was 250 m in diameter and at least 40 m high, putting the top about 60 m above the bottom of the main crater floor. The volume was an estimated 1.96 million cubic meters. They also witnessed a small ash explosion from the inner crater (figure 71).

Figure 71. The summit crater with dome 55 at Popocatépetl on 27 February 2015. The dome at the bottom of the inner crater was estimated to be 250 m in diameter and at least 40 m high (upper). CENAPRED scientists witnessed a small ash explosion from the inner crater during the overflight (lower). Courtesy of CENAPRED.

The Washington VAAC issued reports of ash emissions on 3 February, and during 11-16 and 24-28 February. Ash plumes identified in satellite imagery rose to altitudes of 6.1-6.7 km during 11-13 February and drifted as far as 5 km NE. On 24 February, a plume was seen extending about 15 km ENE from the summit at 6.7 km altitude. The next day an ash plume was observed in satellite imagery at 9.1 km altitude extending NE about 12 km from the summit. Later that day (25 February) it extended 300 km NE at 6.7 km altitude, out over the Gulf of México, before it dissipated. Additional emissions on 25 February occurred about every 60-90 minutes and drifted 130 km ENE at 8.2 km altitude. These bursts of ash continued moving ENE and finally dissipated about 170 km from the volcano. Plumes observed on 27 and 28 February in multispectral satellite images rose to 7-7.9 km altitude. A small area of faint ash from the 27 February emission was visible in images in the Gulf of México about 390 ENE of the summit late on 28 February, while a new emission was visible extending NE about 25 km. Twenty-five MODVOLC thermal alerts were issued most days during February (except 12-17). The OMI instrument on the AURA satellite captured 14 days of SO2 emissions with DU>2.

Activity continued at a high rate during March 2015, again with hundreds of emission events with gas, steam, and small quantities of ash (figure 72). Larger quantities of ash from multiple-per-week explosions rose 1-3 km above the summit and drifted N or NE. Incandescent tephra was ejected 100-800 m onto the N, NE, and SE flanks at least four times. A series of explosions on 7 March led to ashfall reported in Ecatzingo (15 km SW). On 9 March ashfall was reported in Amecameca (20 km NW), Ecatzingo (15 km SW), and Tepextlipa from explosions the previous day. A four-hour series of explosions on 24 March produced steam, gas, and ash emissions that rose 3 km.

The Washington VAAC reported ash emissions every day during 1-5, 7-10, 19-21, and 24-26 March. During the first week, the plumes rose 6.1-7.6 km altitude, drifted NE, N, and NW, and were usually visible for about 100 km from the summit before dissipating. On 8 March, two plumes drifted in opposite directions: one went 15 km ENE at 7 km altitude and one drifted 45 km W at 5.6 km altitude. During the second half of March, the plumes drifted generally NE, at altitudes of 6.1-7.3 km, tens of kilometers before dissipating. Only 11 MODVOLC thermal alerts were issued in March; SO2 data showed four days with DU>2, although SO2 plumes were visible in satellite data almost every day.

Hundreds of daily ash emissions were noted by CENAPRED during April 2015. Ash plumes generally drifted N or NE at 1-3 km above the summit crater, but occasionally they drifted W or SW. Incandescence was often noted at night. Incandescent tephra was ejected several hundred meters onto the flanks during 4-6 April, and again on 18 and 20 April. The only ashfall reported during the month was in Tetela and Ocuituco (both about 22 km SW) after ash-bearing explosions during 3-4 April.

During an overflight on 10 April (figure 73), scientists confirmed that a lava dome had been emplaced in the bottom of the crater between 24 March and 4 April. The lava dome was at least 250 m in diameter and 30 m high. The surface of the dome had concentric fractures and the central part was collapsed from deflation.

Figure 73. During an overflight on 10 April 2015, CENAPRED scientists confirmed that a lava dome had been emplaced in the bottom of the inner summit crater at Popocatépetl between 24 March and 4 April. Courtesy of CENAPRED.

The Washington VAAC issued aviation alerts during 1, 3-8, 13, and 18-21 April. On 3 April volcanic ash was observed moving SE from the summit at 8.2 km altitude. The plume extended over 150 km before dissipating later in the day. Another plume the same day rose to 9.1 km altitude and drifted 55 km NE. During 4 and 5 April, ongoing emissions at various altitudes from 6.1 to 9.1 km drifted in multiple directions for tens of kilometers before dissipating. Most of the alerts were for brief, intermittent emissions that dissipated within 20 km of the summit after a few hours. On 7 April one ash cloud drifted 45 km SSE and another drifted 100 km SE, both at 7.6 km altitude. An ash emission on 13 April traveled around 260 km E at 7.3 km altitude before dissipating. The plumes observed during 18-21 April ranged from 6.7 to 9.7 km in altitude, and mostly drifted NE or E. There were 20 MODVOLC thermal alerts issued during April, scattered throughout the month. Most days during April had SO2 plumes with values >2 DU in the satellite data.

Ashfall was reported in San Pedro Benito Juárez (10-12 km SE), in the municipality of Atlixco Puebla on 2 May 2015, and in Ocuituco (24 km SW) on 22 May. On 26 May ashfall was reported in Tetela del Volcán (20 km SW) and slight ashfall was recorded in Amozoc, Puebla (60 km E) on 31 May. The ongoing explosions generated ash emissions that generally rose 0.5-2.5 km above the crater rim and sent plumes to the SW, SE, and E (figure 74). Nighttime crater incandescence was observed on most clear nights.

Although aviation alerts from the Washington VAAC were issued during 9 days of May (2, 10, 20-21, 25-26, 28, and 30-31), plumes were only visible in satellite images a few times. The highest plume was on 20 May, at 8.2 km altitude drifting SSW. The plume on 26 May was observed drifting NW at 6.1 km, extending 150 km from the summit. Only four MODVOLC thermal alerts were issued during 10, 19, 21 and 30 May, but strong SO2 plumes (>2 DU values) were recorded 12 times, with just as many days showing smaller-magnitude plumes.

Activity was much quieter at Popocatépetl during June 2015. Only six VAAC reports were issued (during 7-8, 12, and 21-22), and only two were identified in satellite images. The plume on 7 June rose to 8.2 km altitude and drifted SW. The larger plume on 12 June came from multiple small emissions; it rose to 6.1 km altitude and was last seen at 55 km SW of the summit before dissipating. There were seven MODVOLC thermal alerts on seven different days during June, and 17 different days with SO2 plumes with recorded values >2 Dobson Units.

Activity during July-December 2015. Multiple daily emissions, nighttime incandescence, and intermittent explosions continued during July 2015 (figure 75). Nine MODVOLC thermal alerts were issued, but they were concentrated during 6-8 and 26-31 July. The Washington VAAC issued alerts on 8, 10, and 11 July, and then during a second period from 24 to 28 July. The report on 8 July noted an ash emission at 7.6 km altitude extending 15 km SW from the summit. The report on 10 July noted that ashfall had been reported about 10 km NW of the summit, but cloudy skies prevented satellite observations. Reports issued during 24-28 July included satellite observations of emissions at 6.1 to 7.6 km altitude extending 25-45 km NE or W from the summit before dissipating. The SO2 emissions during July were visible nearly every day in the satellite data, with 16 days having values >2 DU.

Sulfur dioxide emissions during August 2015 were also visible in satellite imagery nearly every day. Six days had values >2 DU. There were no Washington VAAC reports during August, but there were ten MODVOLC thermal alerts issued throughout the month.

The number of daily emissions during September 2015 were far fewer than during January-July 2015, although crater incandescence was still observed. The Washington VAAC only issued three reports, all during 19-20 September. They observed an ash emission on 19 September at 6.7 km altitude that extended 45 km WNW from the summit for a few hours before dissipating (figure 76). Ten MODVOLC thermal alerts were issued in September, and SO2 plumes were visible daily with values >2 DU on half the days of the month.

Ash emissions increased again during October 2015. Ash-bearing plumes rose as high as 2 km above the crater. The Washington VAAC issued reports of ash plumes on 12 different days. An ash plume observed on 2 October at 7.6 km altitude extended 185 km SW before dissipating; another plume on 20 October was identified in satellite images at 8.5 km altitude drifting NW, and was visible from México City. Eighteen MODVOLC thermal alerts were issued throughout the month, and strong SO2 plumes were detected nearly every day in OMI satellite data.

Activity during November 2015 was similar to that during October. CENAPRED recorded tens of daily emissions of water vapor, gas, and minor amounts of ash. Explosions at regular intervals sent ash plumes 1-3 km above the summit, and incandescent material was deposited on the flanks within 1 km of the crater a number of times (figure 77). The Washington VAAC issued aviation alerts almost daily during 1-17 November, but none after that for the rest of the year. Most of the ash plumes reached 6.1-7.3 km altitude and drifted N, NE, SW, W, and S for a few tens of kilometers before dissipating. The plume on 7 November rose to 9.1 km and was visible as a dark feature above the weather clouds before it dissipated.

Figure 77. Incandescent material showers the flanks of Popocatépetl from an explosion during the early morning hours of 17 November 2015. Webcam image courtesy of CENAPRED.

While ash plume observations decreased during the second half of November and during December, MODVOLC thermal alerts increased in number. Thirty-three appeared during November, and 35 during December. Plumes of SO2 were persistently visible in Aura/OMI satellite data both months.

Activity during January-June 2016. A series of explosive events during 2-8 January 2016 resulted in 13 aviation alerts from the Washington VAAC. An ash plume first reported in satellite data early on 6 January was drifting E at 6.4 km altitude. By late the next day, VAAC reports indicated that the plume was still visible over 1,000 km E before it finally dissipated. A new series of explosive events began on 20 January (figure 78) and lasted through 26 January.

CENAPRED reported that on 23 January 2016 an increase in activity was characterized by continuous gas-and-ash emissions, likely related to the destruction of a recently-formed lava dome. Later that night cameras recorded incandescent fragments ejected during periods of emissions. The constant steam-and-ash emissions drifted E and ENE for more than 48 hours at altitudes from 6.1 to 8.2 km. By 25 January, an ash plume was still visible over 900 km E. NASA Earth Observatory posted a satellite image of the plume around 1930 UTC (1330 local time) (figure 79). NASA's Goddard Space Flight Center also captured an image of a strong SO2 plume drifting NE from Popocatépetl at the same time (figure 80). Twenty-six MODVOLC thermal alerts were issued on 15 days of January. Especially strong SO2 plumes were visible on 6, 7, 23, and 25 January.

Figure 79. Popocatépetl emits an ash plume on 25 January 2016 that extends over 300 km E over the Gulf of México. The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite captured this image at 1930. Image prepared by Jeff Schmaltz, LANCE/EOSDIS Rapid Response using VIIRS data from the Suomi National Polar-orbiting Partnership. Suomi NPP is the result of a partnership between NASA, the National Oceanic and Atmospheric Administration, and the Department of Defense. Courtesy of NASA Earth Observatory.

Figure 80. A strong SO2 plume drifting over 500 km NE from Popocatépetl was captured by the OMI instrument on the AURA satellite during 1918-2015 UTC on 25 January 2016. A visible infrared image was acquired within this same period (see figure 79). Courtesy of NASA GSFC.

Tens of daily emissions of water vapor, gas, and ash were reported during February 2016, along with multiple daily explosions generating ash plumes and occasionally sending tephra onto the flank. The Washington VAAC issued aviation alerts on twelve days during the month. They were discrete events that sent ash plumes generally E or SE at altitudes between 6 and 7 km, and generally dissipated within 6 hours, tens of kilometers from the summit. An ash plume reported on 15 February was still visible 500 km E of the summit before it dissipated. MODVOLC thermal alerts were reported on 10 days during the month, SO2 plumes were more intermittent and only exceeded 2 DU on four days.

The largest ash explosion events during March 2016 took place at the end of the month. On 27 March, an ash plume was spotted by the Washington VAAC extending about 100 km NE at 6.4 km altitude. Explosions on 29 March created an ash plume at 9.1 km altitude moving rapidly ESE (figure 81). Ashfall from the plume caused Puebla's airport to close from 2000 on 29 March to 0600 on 30 March. The plume fanned out and extended tens of kilometers to both the S and SE before dissipating. On 31 March an explosion produced an ash plume that rose 1.8 km and drifted ENE; incandescent fragments fell 1 km away on the ESE flank. Thermal alerts were issued by MODVOLC on 13 days of March, and SO2 plumes were visible about the same number of days, but values did not exceed 2 DU.

Figure 81. An ash plume at Popocatépetl on 29 March 2016. Webcam image courtesy of CENAPRED.

On 2 April 2016 CENAPRED scientists conducted an overflight of the crater and observed the inner crater which was 325 m in diameter and 50 m deep (figure 82). The crater had previously been filled with a lava dome, destroyed in January, which had grown to an estimated volume of 2,000,000 cubic meters. Small landslides had occurred on the E wall of the inner crater. During 3 April, incandescent fragments were ejected as far as 3.5 km onto the E and SE flanks, generating fires in that part of the forest; authorities noted that the event was the largest explosion in three years. Ash fell in the towns of Juan C. Bonilla (32 km ENE) and Coronango (35 km ENE), both in the state of Puebla. The Washington VAAC reported numerous ash plumes during 1-9 April. The highest, on 1 April, was observed in satellite data at 9.7 km altitude, extending over 300 km NE over the Gulf of México. The other plumes were mostly observed between 6.4 and 8.5 km altitude, drifting E or NE.

Figure 82. The inner crater at Popocatépetl on 2 April 2016. CENAPRED scientists estimated that it was 325 m in diameter and 50 m deep. The previous lava dome was destroyed during January 2016. Courtesy of CENAPRED.

Strombolian activity on 18 April ejected incandescent fragments 1.6 km onto the NE flank, and ash plumes rose 3 km above the crater and drifted ENE. Ashfall was reported in San Pedro Benito Juárez (12 km SE), San Nicolás de los Ranchos (15 km ENE), Tianguismanalco (17 km E), San Martín Texmelucan (35 km NNE), and Huejotzingo (27 km NE). According to a news article, the airport in Puebla closed again due to the ash plumes. Thermal alerts from MODVOLC were recorded on 13 days during the month, and SO2 plumes were visible in the Aura/OMI data almost every day.

Activity continued at slightly lower levels during May 2016 with VAAC reports issued on nine days. The ash plumes reported all dissipated quickly within a few tens of kilometers of the summit after drifting E at altitudes generally around 6.4 to 6.7 km. Single MODVOLC alerts were reported on only six days during the month, and except for a large SO2 plume on 3 May, small plumes were visible about 8 days of the month.

An increase in the number of daily explosions with ash emissions was reported by CENAPRED during June 2016. As many as six a day were reported during the second week of the month. An explosion on 12 June produced an ash plume that rose 2.5 km and drifted W (figure 83). Minor amounts of ash fell in Ozumba (18 km W). Aviation alerts were issued by the Washington VAAC on 13 days. Most of the ash plumes dissipated within six hours a few tens of kilometers from the summit due to high winds. The plumes rose to altitudes between 6.1 and 7.9 km, and drifted NE, W and SW. The ash plume reported on 23 June extended NE 16 km at 7.3 km altitude, and 26 km SW at 5.8 km altitude simultaneously. Thermal alerts from the MODVOLC system were reported on 1, 8, and 25 June. SO2 satellite data was only available for the second half of the month, and showed two days with significant SO2 plumes.

Frequent historical eruptions have occurred since pre-Columbian time at México's Popocatépetl. More recently, activity picked up in the mid-1990s after about 50 years of quiescence. The current eruption, which has been ongoing since January 2005, has included frequent ash plumes rising generally 1-4 km above the 5.4-km-elevation summit, and numerous episodes of lava-dome growth and destruction within the 500-m-wide summit caldera. Multiple emissions of steam and gas occur daily, many contain small amounts of ash. Larger, more explosive events that generate ashfall in neighboring communities usually occur every month or two. Information about Popocatépetl comes from daily reports provided by México's Centro Nacional de Prevención de Desastres (CENAPRED). Many ash emissions are also reported by the Washington Volcanic Ash Advisory Center (VAAC). Satellite visible and thermal imagery and SO2 data also provide important observations. Activity through June 2016 was typical of the ongoing eruption with near-constant emissions of water vapor, gas, and ash, and at least two episodes of dome growth and destruction (BGVN 42:07). This report covers similar activity through July 2017.

Activity at Popocatépetl during July 2016-July 2017 was typical of the ongoing eruption since 2005. Near constant steam-and-gas emissions often contained minor amounts of ash. Explosions with ash plumes occurred several times a week during most months. Incandescence at the summit was usually visible on clear nights; nighttime explosions revealed incandescent blocks travelling 100 m or more down the flanks. Large ash explosions on 25 and 26 November 2016 sent ash plumes to 11.5 and 10.9 km altitude, respectively. Ashfall was reported from communities within about 35 km on five different occasions. Thermal activity slowly increased during 2016 to high levels indicative of dome growth during December 2016 and January 2017; they diminished early in 2017 and then fluctuated through July 2017. Sulfur dioxide plumes were persistent in satellite data with plume densities generally exceeding two Dobson Units (DU) several times each month.

Activity during July-September 2016. Intermittent activity continued during July 2016. Tens of daily emissions of gas and steam were reported during the first and last weeks of the month; explosions with ash plumes also generally occurred daily during those weeks, and incandescence was visible on clear nights. The Washington VAAC reported ash emissions observed on 3 July at 7.9 km altitude drifting W, 2.5 km above the summit. Explosions on 4 July produced ash plumes that rose to 8.5 km altitude, just over 3 km above the crater and drifted WSW. Ashfall was reported in Atlatlahucan (30 km WSW) and Tepetlixpa (20 km W). Continuous ash emissions rising to just under 6 km altitude were seen in satellite imagery on 10 July extending almost 40 km W from the summit. Multiple daily explosions occurred during 24-26 July (figure 84). A small cloud of volcanic ash was centered about 35 km W of the summit early on 25 July at 5.8 km altitude. Later in the day, another plume was observed at 7 km altitude drifting WNW and then WSW before dissipating. Satellite imagery confirmed an ash emission on 30 July that rose to 6.7 km altitude and drifted W-WSW. MODVOLC thermal alerts were issued on 3, 10 (2) and 27 July. Small SO2 plumes were recorded daily by the Aura instrument on the OMI satellite. Most measured around two Dobson Units (DU) with an area of about 100,000 km2.

Figure 84. An ash plume at Popocatépetl drifts W from the summit on 25 July 2016 as captured by the ALTZOMONI CENAPRED webcam located about 10 km N of the volcano. Courtesy of CENAPRED.

Tens of daily emissions were common during August 2016, some of which contained minor amounts of ash. Six landslides were detected by the seismic network on 11 August (figure 85). The two largest had volumes of 440 and 220 m3. The Washington VAAC reported an ash plume moving NW at 7.3 km altitude just after midnight on 1 August, extending about 35 km from the summit. A short while later, continuous emissions were reported extending up to 75 km W at 6.1 km altitude. By 0910 UTC, they extended 220 km WNW at 7.3 km altitude. The edge of the plume farthest from the summit had reached close to 500 km W by 1945 UTC when it was last observed. The webcam captured an emission on 11 August but it was not visible in satellite imagery due to weather clouds. An explosion on 12 August generated an ash plume that rose 2.5 km above the 5.4-km-high summit crater and drifted WNW, causing ashfall in Ozumba (18 km W) and Atlautla (16 km W). An explosion at 0034 on 13 August ejected incandescent material onto the flanks. An ash emission was seen in satellite imagery at 8.2 km altitude about 35 km W of the summit later in the morning. Another ash emission was observed with the webcam midday on 15 August that produced an ash plume that rose to 8.5 km altitude and drifted WSW. An ash plume on 21 August drifted W at 6.1 km altitude, and one on 28 August was observed in satellite imagery moving NNW below 7.3 km altitude. Two explosions on 27 August at 1505 and 1537, and one at 0559 on 28 August sent incandescent fragments down the flanks.

Figure 85. A landslide on 11 August 2016 at Popocatépetl was captured by the Tlamacas CENAPRED Webcam located about 5 km N of the summit. Courtesy of CENAPRED.

MODVOLC thermal alerts were issued on 1 (4), 3, 4, 11 (2), 13, and 29 August 2016. SO2 plumes were also captured daily by the Aura Instrument on the OMI satellite, with similar values to those recorded during July. During an overflight of Popocatépetl on 30 August 2016 CENAPRED scientists confirmed that explosions during 27-28 August had destroyed lava dome 69 (first identified on 1 August). The crater which had hosted the dome was 300 m in diameter and 30 m deep (figure 86).

Figure 86. An overflight of Popocatépetl on 30 August 2016 confirmed that explosions during 27-28 August had destroyed lava dome 69 (first identified on 1 August). The crater which had hosted the dome was 300 m in diameter and 30 m deep. Courtesy of CENAPRED.

Only one MODVOLC thermal alert was reported on 14 September 2016. SO2 emissions continued at similar levels to the previous months. Tens of daily steam-and-gas emissions were recorded and crater incandescence was visible on clear nights. An explosion on 8 September produced an ash plume that rose 1.5 km above the crater. On 11 September, an explosion generated a plume that rose 1 km, and an explosion that night ejected incandescent material onto the flanks. CENAPRED reported two volcanic ash emissions on 14 September that rose to 7.3 km. Weather clouds prevented satellite observations of the first, but the second one was observed extending 10 km W of the summit, and reached about 50 km before dissipating. Minor amounts of volcanic ash and steam on 23 September extended NW about 30 km from the summit at 5.5 km altitude. The Mexico City Meteorological Weather Office (MWO) reported volcanic ash at 7.3 km altitude on 29 September, but it was not observed in satellite imagery due to weather clouds.

Activity during October-December 2016. An increase in thermal activity was responsible for ten MODVOLC thermal alerts on 4, 7, 14, 16 (2), 20, 23, 27, 28, and 30 October 2016. Although near-constant steam-and-gas emissions continued, some with minor amounts of ash, there was only one observation of an ash plume from the Washington VAAC, on 28 October at 6.4 km altitude drifting SW. Sulfur dioxide emissions appeared to decrease in the Aura satellite data, although there were values measured over two DU at least four days of the month. Fewer ash emissions were reported during November 2016 as well, but ten MODVOLC thermal alerts were reported on 5, 14, 24, 25, 26 (2), 28, and 30 (3) November.

Ash emissions increased significantly during the last week of November. An ash plume at 5.5 km altitude was visible 55 km E of the summit on 24 November. A larger emission on 25 November was observed at 9.1 km altitude towering above the summit and drifting N (figure 87) with additional ash emissions at 7.3 km altitude drifting SE. Ashfall was reported from this event in areas downwind, including in the municipalities of Atlixco (25 km SE), Tochimilco (15 km SSE), and San Pedro Benito Juárez (12 km SE). Emissions were observed as high as 11.5 km altitude drifting NE later in the day; they continued drifting ENE at 7.9 km into the next day before dissipating 250 km from the summit. A new ash emission on 26 November rose to 10.9 km altitude. It was visible 300 km S of the summit while a second ash cloud was centered 150 km S at 5.2 km altitude. Later in the day, an ash emission was observed at 6.7 km drifting SW.

Figure 87. An ash plume at Popocatépetl rises toward 9.1 km altitude on 25 November 2016 as viewed from CENAPRED'S Altzomoni WEBCAM, located about 10 km N of the summit. Courtesy of CENAPRED.

During 28-29 November 2016 there was another pulse of activity with 48 detected emissions. Beginning at 0559 on 28 November, water vapor, gas, and ash emissions became constant, rising as high as 1.5 km above the crater rim and drifting NE. Incandescent fragments were ejected 300-800 m from the crater, mainly onto the NE flank during the next night (figure 88). Ash fell in Atlixco, Chiautzingo (25 km NE), Domingo Arenas (22 km NE), Huejotzingo (27 km NE), Juan C. Bonilla (33 km NE), San Andrés Calpan (18 km NE), and San Martín Texmelucan (Puebla state, 35 km NNE), and in San Miguel (Tlaxcala state). Plumes from these emissions were reported on 29 November at 7.3 km altitude, and they drifted as far as 170 km NE; remnant ash was observed over the Gulf of Mexico on 30 November. Emission intensity increased again on 30 November and a new continuous plume at 6.4 km altitude extended 370 km NE before dissipating. At 1500 UTC on 30 November, a second higher plume was reported by the Washington VAAC at 9.3 km altitude centered 400 km NE of the summit. The continuous emissions became intermittent on 1 December; the last of the emissions dissipated about 200 km NE of the summit. A short puff noted on the webcam late on 1 December was the last VAAC report for 2016.

Figure 88. Incandescent fragments were ejected 300-800 m onto the NE flank of Popocatépetl on 29 November 2016, as seen from the Tlamacas webcam located about 5 km N of the volcano. Courtesy of CENAPRED.

While ash emissions decreased during December 2016, steam-and-gas emissions continued, and thermal activity increased. MODVOLC alerts were reported 24 times on 17 different days. More substantial SO2 plumes than seen in previous months were also captured by the Aura satellite instrument on 8 and 31 December (figure 89). A new lava dome (71) first detected by CENAPRED on 29 and 30 November had almost completely filled the internal crater by 12 December (figure 90), reaching 280 m in diameter and 50 m thick. The volume of the dome was estimated to be about 3 million m3.

Figure 89. The Aura Instrument on the OMI satellite captured substantial SO2 plumes from Popocatépetl on 8 (top) and 31 (bottom) December 2016. The plume on 8 December drifted NE for several hundred kilometers, and had a maximum DU (Dobson Unit) value of 6.02. The plume on 31 December drifted N and then NE a similar distance and recorded a DU value of 4.91. Courtesy of NASA Goddard Space Flight Center.

Figure 90. A new lava dome was photographed at the summit of Popocatépetl during an overflight on 12 December 2016. Courtesy of CENAPRED.

Activity during January-April 2017. Low-intensity steam-and-gas emissions continued during January 2017; incandescence was regularly observed at the summit. During January, only one emission was reported by the Washington VAAC, on 23 January at 7.6 km altitude drifting NW. They noted that the satellite imagery indicated the emission was mostly gas and water vapor with minor amounts of ash. Thermal activity continued to increase in January 2017 with 35 alerts reported on 26 days of the month. The increase in thermal activity was also visible in the MIROVA log radiative power information plotted from the MODIS thermal anomaly data (figure 91). SO2 plumes were also notable through 18 January, after which they decreased in both size and density in satellite data.

Figure 91. MIROVA log radiative power data for Popocatépetl for the 12 months leading up to 4 August 2017. The increase in thermal activity beginning in late November 2016 corresponds to observations by CENAPRED of the growth of a new lava dome at the summit. Courtesy of MIROVA.

Ash emissions were reported on three days during February 2017. A plume on 7 February rose to 5.8 km altitude and drifted W, dissipating quickly. A plume on 12 February was reported at 6.1 km altitude drifting 10 km N of the summit. An ash emission was recorded on the CENAPRED webcam on 15 February (figure 92); it was seen in satellite imagery at 6.7 km altitude drifting NE, and dissipated after about six hours. Thermal activity decreased during February relative to January. MODVOLC only reported 16 thermal alerts on 11 days of the month.

Figure 92. An ash emission from Popocatépetl on 15 February 2017 was later observed in satellite imagery at 6.7 km altitude drifting NE. Image taken by the Altzomoni webcam, located about 10 km N of the volcano. Courtesy of CENAPRED.

Continued steam-and-gas emissions during March 2017 were accompanied by a few ash-bearing explosions. The webcam captured an ash emission on 8 March that the Washington VAAC observed in satellite imagery drifting N at 5.8 km altitude. Another emission late on 11 March rose to 6.1 km and drifted E; it contained mostly gas with only small amounts of ash. Late on 28 March, an ash cloud was observed in satellite imagery centered about 50 km ENE of the summit at 5.8 km altitude. Thermal activity continued to decrease with only ten MODVOLC thermal alerts issued on six different days during March.

Thermal alerts were fewer still during April 2017; one appeared on 6 April, and then a cluster of six were reported during 24-30 April. Ash emissions were reported by the Washington VAAC on 16, 20, 24, and 25 April. Constant emissions were seen by the webcam on 16 April; they likely contained ash, and were estimated to be at 6.1 km altitude. A small puff of ash was seen in satellite imagery on 20 April drifting S to about 35 km at the same altitude. Multiple emissions of ash mixed with steam and gas were observed in satellite imagery on 24 April moving SE at 5.6 km altitude. Constant steam-and-gas emissions continued throughout the month, with incandescence visible on clear nights. Tephra from explosions on 26 and 27 April was ejected 100 m NE of the crater.

Activity during May-July 2017. Thermal activity increased somewhat during May 2017. Seventeen MODVOLC alerts were reported on 13 different days. SO2 emissions with DU values greater than two occurred eight times during the month. Low intensity explosions with water vapor, gas, and ash emissions occurred daily throughout the month. On 18 May, the Washington VAAC reported an ash plume at 7.3 km altitude drifting N, and they observed a bright hotspot in shortwave imagery. Multiple emissions were later observed, with plumes rising to 7.6 km, moving NNE 70 km from the summit. A small puff of ash was seen in satellite imagery on 21 May at 7 km altitude approximately 25 km from the summit moving N. The leading edge of a new emission was observed the next day about 45 km SSW of the summit at 6.1 km altitude. An ash emission was observed on the CENAPRED webcam on 30 May, but weather clouds obscured any satellite observations.

MODVOLC thermal alerts were reported on 6, 16, 17, 18(3) and 21 June 2017. Observers noted material being ejected 200 m from the crater on 3 June. Cloud cover obscured satellite and webcam views of a reported ash plume on 12 June. A small ash emission was reported on 13 June 16 km W of the summit at 7.0 km altitude.

A series of ash emissions were reported by the Washington VAAC almost daily during 2-11 July 2017. A social media post by CENAPRED and a webcam image showed an ash emission (figure 93) on 2 July. It was observed by the Washington VAAC in satellite imagery moving SW at 6.7 km altitude. Minor ashfall on 2 July was also noted in Ozumba (18 km W), Amecameca (19 km NW), Tlalmanalco (26 km NW), Chalco (38 km NW), Ayapango (22 km NW), Tenango del Aire (28 km NW), and San Pedro Nexapa (14 km NW). An emission on 3 July was confirmed in visible satellite imagery drifting WNW at 6.7 km altitude. On 4 July, a plume was observed in satellite imagery 25 km NE of the summit at 7.6 km altitude. An ash emission observed in the webcam early on 6 July was not visible in satellite imagery due to weather clouds, but a larger emission that evening was spotted with difficulty at 7.6 km altitude, in spite of the weather clouds. CENAPRED reported that the plume was clearly visible nearly 2 km above the summit. The next day, clouds obscured the summit from the webcam, but an ash emission was clearly visible in satellite imagery at 7.6 km altitude moving NW. The webcam recorded additional emissions on 9 and 11 July; they were obscured from satellite images by weather clouds. A plume of mostly gas and steam with a small amount of ash near the summit extended 55 km W of the summit on 31 July at 6.4 km altitude.

Figure 93. An ash emission at Popocatépetl on 2 July 2017 as seen from the Altzomoni webcam, 10 km N of the volcano. Courtesy of CENAPRED.

MODVOLC thermal alerts were reported on 13-15, 21, 23 (2) and 28 July. Sulfur dioxide plumes with densities between one and two Dobson Units were captured by the Aura Instrument on the OMI satellite almost every day of the month.

Ongoing steam, gas, and ash emissions along with intermittent explosions, August 2017-February 2018

Located 60 km SE of Mexico City, frequent historical eruptions have been reported from Popocatépetl going back to the 14th century. Activity increased in the mid-1990s after about 50 years of quiescence, and the current eruption, which has been ongoing since January 2005, has included frequent ash plumes and numerous episodes of lava-dome growth and destruction within the 500-m-wide summit caldera. Multiple emissions of steam and gas occur daily, rising generally 1-4 km above the 5.4-km-elevation summit; many contain small amounts of ash. Larger, more explosive events that generate ashfall in neighboring communities often occur every week.

Activity through July 2017 was typical of the ongoing eruption with near-constant emissions of water vapor, gas, and minor ash, as well as multiple explosions every week with ash-plumes and incandescent blocks sent down the flanks (BGVN 42:09). This report covers similar activity through February 2018. Information about Popocatépetl comes from daily reports provided by México's Centro Nacional de Prevención de Desastres (CENAPRED); ash emissions are also reported by the Washington Volcanic Ash Advisory Center (VAAC). Satellite visible and thermal imagery and SO2 data also provide important observations.

Near-constant emissions of steam and gas, often with minor ash content, were typical activity for throughout August 2017-February 2018. Intermittent larger explosions with plumes of moderate ash content that generated ashfall in nearby communities were reported in most months, including several times during October and November 2017, reaching communities as far as 70 km away. Incandescence at the summit was often observed on clear nights, and Strombolian activity that sent incandescent blocks several hundred meters down the flanks occurred at least once each month during September 2017-January 2018. The tallest ash plumes during the period reached 9.1 km altitude in mid-October and 10.3 km altitude at the end of January 2018. Thermal anomalies were persistently detected in satellite data throughout the period, and SO2 plumes were recorded every month with satellite instruments.

Activity during August-September 2017. The Washington VAAC reported satellite observations of an ash plume extending 55 km W of the summit at 6.4 km altitude on 31 July 2017; the plume was mostly gas and steam with a small amount of ash. CENAPRED reported ashfall in Ozumba (18 km W) on 1 August from a plume that rose 2 km above the summit. They also noted numerous low-intensity explosions with steam, gas, and ash during 5-7 August. A small explosion early on 14 August produced a 500-m-high plume with minor ash content that drifted SW. Two explosions later in the day generated ash plumes that rose 0.8 and 1.5 km from the summit and drifted W (figure 94). Another explosion on 15 August produced a plume over 1 km in height with moderate ash content. On 21 August CENAPRED reported an ash plume that rose 4 km and drifted NW (figure 95). The Washington VAAC reported this plume extending 33 km W from the summit at 7.6 km altitude. Later in the day the ash cloud was observed about 230 km W of the summit, and a new cloud at a slightly lower altitude had drifted 45 km NW.

Figure 94. An ash plume drifted W from Popocatépetl on 14 August 2017 as seen from the Tlamacas webcam located about 5 km N of the volcano. Courtesy of CENAPRED.

Figure 95. An ash plume at Popocatépetl rose 4 km above the summit on 21 August 2017 and drifted over 200 km W before dissipating. View is from the Altzomoni webcam, located about 10 km N of the summit. Courtesy of CENAPRED.

CENAPRED noted 22 explosions with ash during 25-26 August that drifted N and NW. They were observed in satellite imagery by the Washington VAAC at 7.6 km altitude. Eleven explosions with small amounts of ash were reported by CENAPRED on 27 August. There were daily explosions during 28-31 August, but weather clouds obscured views of the summit. Incandescence at the summit crater was observed on many clear nights during August.

During 1-11 September 2017 cloudy conditions generally prohibited observations of the summit, but low-intensity emissions of steam and gas were briefly observed, many containing minor ash. Five explosions with minor ash emissions were reported by CENAPRED on 12 September; the Washington VAAC noted the ash plume in satellite imagery at 6.7 km altitude drifting slowly N. CENAPRED reported 22 explosions with ash and incandescent rocks on the NE flank during 12-13 September.

The Washington VAAC reported ash plumes on 13 September at 8.2 km altitude, on 18 September at 6.4 km altitude drifting W, and on 23 September near 7 km altitude moving to the NNE. Numerous explosions were reported by CENAPRED during 27 and 28 September (figure 96). The Washington VAAC reported the dense ash plume from these explosions at 6.7 km altitude drifting WSW. It extended 130 km W of the volcano by early afternoon on 27 September. CENAPRED reported that an explosion late on 30 September sent incandescent fragments 0.8 km from the crater and produced a dense ash column that rose more than 2 km above the summit.

Figure 96. A dense ash emission from Popocatépetl on 27 September 2017 extended 130 km W before dissipating as viewed from the Altzomoni webcam, located about 10 km N of the summit. Courtesy of CENAPRED.

Activity during October-November 2017. The ash plume from the explosion late on 30 September 2017 was visible in satellite imagery the following morning located 15 km SW from the summit at 7.9 km altitude according to the Washington VAAC. CENAPRED reported three explosions on 2 October and five explosions the next day, causing ashfall in Atlautla (17 km W), Tepetlixpa (21 km W), and Ozumba. Three explosions on 5 October resulted in ashfall in Totolapan (32 km W), Tlalnepantla (40 km W), and Cuernavaca (64 km W), and closer to the volcano in Ecatzingo (15 km SW), Atlautla, and Tepetlixpa. Lahars were also observed on the W flank, but there were no reports of damage. Two more explosions on 6 October led to ashfall reported from Zacualpan de Amilpas (30 km SW) and Tetela del volcán (18 km SW) (figure 97). The Washington VAAC reported the 6 October emissions at 6.4 km altitude.

Figure 97. Webcam image showing one of the two explosions on 6 October 2017 at Popocatépetl that caused ashfall in Zacualpan de Amilpas (30 km SW) and Tetela del volcán (18 km SW). The Tlamacas webcam is located about 5 km N of the volcano. Courtesy of CENAPRED.

The first of two explosions on 7 October 2017, shortly after midnight, produced a plume that rose over 2 km and drifted SW with ashfall reported in Tetela del volcán; incandescent blocks were also sent down the flanks (figure 98). The second explosion produced an ash plume that rose 3 km and drifted NNE. The Washington VAAC reported continuing ash emissions during 7-11 October. Numerous plumes rose to 5.8-9.1 km altitude and drifted in several different directions; the plume extended 130 km SW from the summit on 10 October. CENAPRED reported three explosions on 8 October (figure 99) and two on 9 October. Numerous low-intensity exhalative events during 10-12 October produced ash plumes less than 1 km above the crater that drifted SW. Ashfall was reported in several communities during this time including Ozumba, México City (60 km NW), Milpa Alta (45 km NW), Xochimilco (56 km NW), Tlalpan (68 km NW), Coyoacán (66 km NW), Iztapalapa (57 km NW), Magdalena Contreras (72 km NW), and Iztacalco (64 km NW).

Figure 98. Incandescent blocks visible in this image traveled down the flanks of Popocatépetl during the early morning of 7 October 2017. The Tlamacas webcam is located about 5 km N of the volcano. Courtesy of CENAPRED.

Figure 99. Multiple explosions from Popocatépetl on 8 October 2017, including the one seen here, caused ashfall in several communities NW of the volcano. The Tlamacas webcam is located about 5 km N of the volcano. Courtesy of CENAPRED.

CENAPRED noted incandescence at the crater during most nights from 14 to 31 October, as well as steam, gas, and minor ash from hundreds of low-intensity emission events each day. The Washington VAAC reported ash emissions visible in satellite imagery on 16, 20-22, and 26 October drifting in several different directions at altitudes of 5.8-7.6 km. The plume observed on 22 October reached 60 km from the summit before dissipating. CENAPRED reported two explosions with ash plumes each day during 25-27 October. The Washington VAAC reported an ash plume on 29 October at 6.1 km altitude drifting E about 35 km from the summit, and another at 6.7 km the following day along with an infrared hotspot visible at the summit.

The Washington VAAC issued multiple daily ash advisories throughout November 2017. CENAPRED reported hundreds of daily low intensity emissions of gas and steam that often contained minor ash; the plumes generally rose about 1 km above the summit and most often drifted SW. They also observed incandescence at the crater on all clear nights. They reported Strombolian activity on 3 November in the early morning that lasted for several hours. Explosions early on 4 November resulted in minor ashfall in Yecapixtla (29 km SW) and Zacualpan de Amilpas and other areas to the SW. A Strombolian episode later that day lasted for about an hour and resulted in minor ashfall in Tetela del Volcán. Another explosion that night sent incandescent fragments 200 m down the flanks.

An explosion on 6 November sent an ash plume 2.5 km above the summit crater that drifted SW and sent incandescent fragments 500 m down the flank. Another explosion during the early morning of 7 November produced a 2-km-high ash plume. Moderate amounts of ash rose 1 km above the summit on 8 November. There were three explosions on 10 November; the largest produced a 3-km-high ash plume that drifted SW. Continuous low-level emission of gas and ash on 14 November resulted in ashfall reported in Totolapan, Yecapixtla, Ocuituco (23 km SW), Tetela del Volcán, and Ecatzingo. An explosion on 17 November sent an ash plume 2.5 km above the summit that drifted SW. During 18-19 November five explosions caused ash plumes to rise 2 km above the summit and incandescent blocks to fall down the E flank.

Around 1030 on 20 November, seismic activity increased and was accompanied by a constant plume of steam, gas, and moderate ash that rose about 1.5 km and drifted E. During 20-21 November eight explosions were reported, with five more the following day. During the afternoon of 23 November a continuous ash emission that lasted 90 minutes drifted SSE at 2 km above the summit, and spread ash over communities to the SSE including Huaquechula (30 km SSE), Tepeojuma (38 km SE), Atlixco (23 km SE), and Izúcar de Matamoros (50 km SE) (figure 100). Another significant ash emission during the afternoon of 24 November sent a column of ash to 4 km above the summit, drifting SSE; it lasted for almost two hours (figure 101). The Washington VAAC reported the plume at 8.5 km altitude. Ashfall was reported in San Pedro Benito Juárez (12 km SE) and Atlixco. Late that evening, an explosion sent incandescent fragments 1 km down the flanks and generated an ash plume that rose to 2.5 km above the summit and also drifted SSE.

Figure 100. A continuous ash emission at Popocatépetl that lasted for 90 minutes drifted SSE at 2 km above the summit, and spread ash over several communities to the SSE on 23 November 2017. The Tlamacas webcam is located about 5 km N of the volcano. Courtesy of CENAPRED.

Figure 101. A substantial ash emission at Popocatépetl during the afternoon of 24 November 2017 sent a column of ash to 4 km above the summit that drifted SSE; it lasted for almost two hours. The Washington VAAC reported the plume at 8.5 km altitude. The Altzomoni webcam is located about 10 km N of the summit. Courtesy of CENAPRED.

A flyover by CENAPRED and the Federal Police on 25 November 2017 allowed evaluation of the changes in the summit crater from the recent explosions. They noted that the internal crater within the summit crater had increased its dimensions, reaching a diameter of 370 m and a depth of 110 m (figure 102). A 3-km-tall ash plume resulted from continuous emissions that began in the afternoon of 27 November and lasted for two hours. The Washington VAAC reported the plume at 7.9 km altitude. The plume drifted SSE, and dispersed ash over communities in that region including Tochimilco (16 km), Izucar de Matamoros, Atlixco, and Huaquechula.

Figure 102. During a flyover on 25 November 2017, CENAPRED observed that the increased size of the internal summit crater at Popocatépetl was 370 m in diameter and 110 m deep. Courtesy of CENAPRED.

Activity during December 2017-February 2018. The Washington VAAC issued multiple daily reports of ash emissions during 1-12 and 24-31 December 2017. CENAPRED noted hundreds of daily low-intensity emissions of gas and steam, most with small quantities of ash, throughout December, as well as multiple ash emissions on many days that rose generally 1-2.5 km above the summit. In the early morning of 2 December an explosion caused an ash plume to rise 2.5 km above the summit. A second plume rose 1 km later that day; they both drifted SSE. An explosion in the afternoon of 9 December sent an ash plume over 2.5 km above the summit that drifted NE. The Washington VAAC reported the plume at 7.6 km altitude. Later that evening Strombolian activity sent incandescent blocks down the flanks and generated an ash plume that drifted E. Incandescence was observed at the summit crater during the nights of 17-21 and 24-29 December. Continuous emissions of steam, gas, and moderate-density ash were reported drifting NW for about 90 minutes on 29 December. An explosion on 31 December at 1032 generated a 2-km-high ash plume that also drifted NW.

There were multiple daily reports of ash emissions issued by the Washington VAAC during most days of January 2018. CENAPRED noted hundreds of daily low-intensity emissions of gas and steam, many with small quantities of ash, throughout the month, as well as explosions with ash emissions on many days that generally rose 1-2.5 km above the summit. They also observed incandescence at the summit crater multiple days each week. Ongoing low-level emissions of steam, gas, and minor ash were reported during 4-5 January. During the evening of 5 January activity increased, and the ash plume rose to 800 m and drifted SE. In addition, incandescent blocks were ejected 200-300 m down the flanks for about two hours.

An explosion on 18 January 2018 generated an ash plume that rose 1.5 km above the summit and drifted E while incandescent blocks were ejected up to 700 m down the flanks. An episode of Strombolian activity in the early morning of 25 January produced an ash plume that rose 2 km above the summit and drifted N and NE, resulting in reports of ashfall in San Pedro Nexapa (14 km NE) and Amecameca (19 km NE). It lasted for about 2 hours. Four explosions were reported during the afternoon of 29 January and an explosion the following afternoon produced an ash plume that rose more than 3 km above the summit, and was dispersed to the NW. An explosion on 31 January also produced a substantial ash plume that the Washington VAAC reported at 10.3 km altitude moving NNE (figure 103).

Figure 103. An ash plume rose to 10.3 km altitude from Popocatépetl on 31 January 2018 and drifted NNE. The Altzomoni webcam is located about 10 km N of the summit. Courtesy of CENAPRED.

Activity was somewhat quieter at Popocatépetl during February 2018. The Washington VAAC reported ash emissions on 14 days during the month. CENAPRED reported tens, not hundreds, of daily low-intensity emissions of gas and steam that often contained minor amounts of ash. They also noted one or more explosions with ash emissions on many days that rose generally 1-1.5 km above the summit and drifted in various directions. During many clear days they observed nearly constant emissions of steam, gas, and minor ash that reached 500-800 m above the summit. An explosion on 20 February produced an ash plume that rose 1.5 km above the summit. Continuous steam and gas emissions during 22-23 February were accompanied by minor incandescence intermittently observed at the summit.

Satellite data. Sulfur dioxide emissions were large enough to be recorded by satellite instruments several times every month during August 2017-February 2018 (figure 104). Variable wind directions and persistent emissions produced relatively long-lived plumes that dispersed over large areas of Mexico.

Thermal anomaly data provided by the MIROVA project are consistent with the visual record of persistent incandescent and explosive activity at the summit (figure 105). Multiple MODVOLC thermal alerts were also recorded every month from October 2017-February 2018.

Figure 105. Thermal anomalies detected by satellite-based MODIS instruments and recorded through the MIROVA project show the pattern of continued moderate-level activity at Popocatépetl during the year ending 12 July 2018. Courtesy of MIROVA.

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.

Synonyms

Xalliquehuac | Popocatzin | Popocatepec

Cones

Feature Name

Feature Type

Elevation

Latitude

Longitude

Ecatzingo Cones

Pyroclastic cone

Fraile, Volcán el

Stratovolcano

Nexpayantla

Stratovolcano

Domes

Feature Name

Feature Type

Elevation

Latitude

Longitude

Tlamacas

Dome

Basic Data

Volcano Number

Last Known Eruption

Elevation

LatitudeLongitude

341090

2018 CE

5393 m / 17694 ft

19.023°N
98.622°W

Volcano Types

Stratovolcano(es) Caldera Pyroclastic cone(s) Lava dome(s)

Rock Types

MajorAndesite / Basaltic AndesiteDaciteBasalt / Picro-Basalt

MinorTrachyandesite / Basaltic Trachyandesite

Tectonic Setting

Subduction zoneContinental crust (> 25 km)

Population

Within 5 kmWithin 10 kmWithin 30 kmWithin 100 km

325
2,584
634,054
26,509,510

Geological Summary

Volcán Popocatépetl, whose name is the Aztec word for smoking mountain, rises 70 km SE of Mexico City to form North America's 2nd-highest volcano. The glacier-clad stratovolcano contains a steep-walled, 400 x 600 m wide crater. The generally symmetrical volcano is modified by the sharp-peaked Ventorrillo on the NW, a remnant of an earlier volcano. At least three previous major cones were destroyed by gravitational failure during the Pleistocene, producing massive debris-avalanche deposits covering broad areas to the south. The modern volcano was constructed south of the late-Pleistocene to Holocene El Fraile cone. Three major Plinian eruptions, the most recent of which took place about 800 CE, have occurred since the mid-Holocene, accompanied by pyroclastic flows and voluminous lahars that swept basins below the volcano. Frequent historical eruptions, first recorded in Aztec codices, have occurred since Pre-Columbian time.

This volcano is located within the Las Volcanes, a UNESCO Biosphere Reserve property.

References

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography.